The motivations for protests against homosexuality differ based on the group, and as such the answers must be tailored to the groups. It should also be mentioned that overall (meaning as a percentage) relatively few Christians tend to take part in these protests, for precisely the reason you state.
Furthermore, there are several Christian denominations that active support homosexuality. The Metropolitan Community Church, for example, is a denomination that is in many ways founded on the idea idea of being gay positive. Other denominations, such as the more liberal parts of the Anglican Communion (i.e. the Episcopal Church in the US) actively support and encourage the lifestyle, although this splits the church.
Because I do not know what group is doing the protest at your school, let me break down the motivations of a few that do.
Hate (e.g. Non-Reflective Christians)
I would argue the largest body of gay-haters are, as you say, Christians who haven't read the Scripture. Remember that for many people, Christianity is a cultural thing, not a spiritual thing. Homosexuals are different (if you'll forgive the pun.) For many people, homosexuality represents a threat to the established order, and if there is one thing many people hate, it is change. For centuries, many "Christians" also "knew" that white people were better than black people. There were even sermons that pointed out how Ham and Caanan (who I guess must have been black) was cursed by God (well, Noah really, but Noah speaking for God, I guess). As such, by reading verses out of context, one could conveniently ignore texts like "In Christ there is neither Jew nor Greek,"... etc...
But what such hatred did do is to serve the preservation of the culture. People who don't want to change will naturally seek preserve it in any means possible. There are "scientific," "cultural," and "historical" arguments. Note: They may even be valid. But the motivation is important, and if it is not done out of a God given spirit, despite the veneer of Christianity, it is not Christian.
It should be said here that Jesus never claimed that "preserving the culture" was his mission. His mother (in Luke 1) exults in the fact that old order will be turned upside down. He himself also says in Luke 12:
49 “I have come to bring fire on the earth, and how I wish it were already kindled! 50 But I have a baptism to undergo, and what constraint I am under until it is completed! 51 Do you think I came to bring peace on earth? No, I tell you, but division. 52 From now on there will be five in one family divided against each other, three against two and two against three. 53 They will be divided, father against son and son against father, mother against daughter and daughter against mother, mother-in-law against daughter-in-law and daughter-in-law against mother-in-law.”
Fear (e.g. Westboro Baptist Church)
Fred Phelps is a minister who believes that the judgment of God is being visited on the world because of homosexuals. He and his church are almost universally condemned across the Christian spectrum. Their preferred method of "being faithful to the Gospel" as they see it is to find high profile, sad events, like military funerals or the funerals of well known people, and hold up signs tht say things like "God hates fags." These people protest out of fear - not necessarily of homosexuals per se - but out of fear that a totally just God will visit punishment on the country. They misread texts like Luke 11:31 saying that the USA is worse than Sodom, and that in the final judgement, God will visit destruction upon us all.
Again, it should be made clear that this is not biblical. As 1 John states (ch 4)
Dear friends, let us love one another, for love comes from God. Everyone who loves has been born of God and knows God. 8 Whoever does not love does not know God, because God is love ... God is love. Whoever lives in love lives in God, and God in them. 17 This is how love is made complete among us so that we will have confidence on the day of judgment: In this world we are like Jesus. 18 There is no fear in love. But perfect love drives out fear, because fear has to do with punishment. The one who fears is not made perfect in love.
In Ancient Rome, there were laws against atheism for this exact same reason - fear that a vengeful god would destroy the town for gross acts of sacriliege by any who would deny the gods' existence. Initially, because Jews and Christians only believed in one God, they were branded atheists and punished accordingly.
Love (e.g. Exodus International)
Finally, there is a Christian motivation for the sentiment, if not the tactic (sorry, protests don't change minds) of persuading people "not to be gay." From the website of Exodus International:
Many settle for so much less than what God intended. Perhaps you are one of them. But the Good News of the Gospel is we can be restored and reconciled to the MORE we were created to experience. God came to bring us life and life MORE abundantly
First off, please understand that Exodus International is extremely controversial. Many, many Christians would disavow it*, but it is worth looking at a motivation that really is grounded biblically. In their estimation, homosexuality damages the individual - it is an oppression from which an individual can and should be freed. Much like an alcoholic should be freed from the prison of drink, so a homosexual, in their estimation, should be freed from the "stunted growth" of a homosexual lifestyle.
*Update: Recently, Exodus International has disavowed itself of any conversion therapy approaches, meaning that it does not believe that any proclivities will necessarily disappear. They continue to stand with gays, however, in resisting the impulse to act on the desires, and do so with love, compassion, and empathy.
Love, it should be remembered, does that which is in the best interests of its target. It is more than being nice. When I stop my son and my daughter from doing things that hurt them, that is love. That is the motivation out of which Exodus International operates, and I argue, the real reason why people hate it so much - because it actually is trying to be Christian. It isn't borne out of fear or hate, nor is it a caricature of faith - it is people who genuinely want to be Christian. People don't hate counterfeit three dollar bills - they hate the genuine thing.
Finally, a personal note. I was engaged in a homosexual relationship for many months after college. I do not claim to speak on behalf of all gays, and I won't even argue that my experience was typical. I will tell you that for me (and I repeat FOR ME) homosexuality was ultimately a trap. I liked the companionship, and the relationship was actually quite positive. For a young man to be involved in a deep romantic relationship for the first time in my life, it was highly pleasurable.
But I am glad I was not allowed to stay in that relationship. I eventually had a heterosexual relationship and even children. The joy of those relationships is, frankly, far more than I could have imagined when I was "stuck" in that homosexual one. Most people go through a homosexual phase, but continue to grow. I, personally, believe that phase holds many people - or at least held me - back. If I can help a person grow, I will.
Tatically speaking, a protest is not the way to do that. Jesus did command Christians to love everyone, and holding a sign is not love. I do not advocate for laws banning homosexuality (whether or not marriage is state thing or a church thing is not this question in any event). And if you ask me, do I hate homosexuals? I can clearly answer no. But will I do everything in my power to help someone who is, in my estimation, trapped? Love compels me to say, Of Course.