What is the Roman Catholic position on the validity of the marriage between an infertile man and infertile woman? Also, from this standpoint, what is the difference between a marriage in which both partners are opposing sexes but infertile and a marriage in which both partners are the same sex?
I assume you are asking about the validity of a marriage between people who cannot conceive children. The position is summed up in this paper. To summarize:
Your last point, about marriage between partners of the same sex, is an entirely different question and I suggest you ask it separately.