What knowledge exactly do you want to convey to them to persuade them that gay marriage is a good thing?

Kind of a weird example, but I'll assume we're talking about the Praetorian Guard. The Romans seem to have had very little respect for women and for being penetrated. So right off the bat, having them read a lot of women's minds might change their views. (I'm not sure if I want to classify that as knowledge, though.) They likely also have false beliefs not only about women but about the gods and stable societies. None of this seems like a cure-all, but it does seem extremely promising.