Well with a group role-play, it might just get confusing to keep track of who is and who isn't okay with having their character paired with the same-sex. Having an only maleXfemale rule can get rid of a lot of confusion and keep any mix-ups away.

Also, what if personA decided that despite the rule, they wanted their character to fall for another of the same-sex? Knowing full well that it's not going to happen could make it seem appealing in the sort of "unattainable love" kind of way, meanwhile the personB is just uncomfortable with the types of posts that are being sent their way.

It's not necessarily a homophobic thing, I mean, I can't exactly speak for this person but I can fathom a reason for the rule. Who knows what people have done in the past? It could very well just be a safety precaution against more foolishness.