1. If you are willing to allow normal marriages where the couple doesn’t intend to have children, or is incapable of having children, you lose one of the principal secular arguments against gay marriage. While there have been cultures where child-free marriages were banned (which would probably mean the woman has to get pregnant before the wedding), no modern western culture treats it that way (halacha certainly doesn’t). Once you have redefined marriage as an expression of “couplehood” rather than an economic framework for raising children, the secular objection to “gay marriage” disappears.
2. Same-sex couples do sometimes raise children, and indeed, some children are raised by other than their natural parents for a variety of reasons. One probably should allow parental rights to such couples (whether or not the couple is “involved” with each other). At present, only marriage does that.
3. Based on halacha, the objection to “gay marriage” is really an objection to male homosexuality, which is a religious based objection. Marriage is just a legal and economic framework. There really isn’t a secular reason to prefer one form of perversion (e.g. non-marital sexual intimacy) over another.