Yes, there would be problems with that.
The Church views "openness to having children [biological children of one's own, i.e. children conceived by the woman and the man]" as a vital part of marriage:
The marriage covenant, by which a man and a woman form with each other an intimate communion of life and love, has been founded and endowed with its own special laws by the Creator. By its very nature it is ordered to the good of the couple, as well as to the generation and education of children.
(This does not mean that if a couple, wanting children, discover after the wedding that they are infertile, their marriage is ipso facto invalid. The willingness and desire to have children of one's own is the key here.)
Homosexuals, however, are of their very nature unable to have biological children of their own (that is, with genetic material of both parents) produced by a natural sexual act. For this reason, a relationship between them is unable to be classified by the Church as a "marriage" as the Catholic Church defines the term.
The fact that the relationship is legalized by an act of the State, not the Church, makes no difference; some in the Church have argued that having the State be the entity that recognizes and regulates marriage actually confuses the issue. Marriage is a relationship instituted by God; and having a relationship which is called "marriage" but is not is at best confusing, and at worst scandalous (i.e. tending to promote sinful activity).
Thus, the Church would say that such a relationship as you describe is not an actual marriage, and the couple involved may harm themselves and perhaps others by pretending to be what they in fact are not.