Firstly, many users don't just use genealogy software to track their DNA ancestors, people use it to record details of their family tree. If a relative died without leaving any offspring, that person isn't deleted from the tree, they are still there. Likewise, people may want to record same sex marriage in their family trees, even though there might not be any off spring. People in same sex marriages might want to track their own family trees. A man might want to include his mother and his mother-in-law (his husband's mother).
Secondly, yes, some same sex marriages can produce children. From adoption, to artificial insemination/surrogacy, to one partner having a child(ren) from a previous marriage, which the other adopts, etc.
Thirdly, it is possible to have genetic children from a same sex marriage. Some countries have gender recognition law which allows trans people to legally change their gender and get a new birth cert. Depending on the law, they may still be in the original marriage, and still legally have children. (Yes there are people who start living as a different gender and their spouse still loves them and stays with them.)
Yes, these are new issues, and a lot of genealogy deals with stuff in the past, but this is only going to get more common as time passes. Blanket statements saying essentially "This is nothing to ever be worried about at all" is false.
(As an aside, bisexuals et al. can enter into same sex marriages, it not just 'homosexual marriage')
