I don't think it's that kind of insecurity to be honest. I think it's social insecurity, unless you're talking about those leaders of ex-gay ministries who are the most closeted s.o.b.s on the planet. Anyway, people have this idea of normalcy, anything that goes against that is disturbing.This is true of anything that upsets what they believe to be the proper order of things. They also resent the perceived special attention these groups are getting without remembering how much privilege they actually have.
Also, many guys, myself included, think girl on girl is hot. Thus lesbians are immune to many forms of homophobia.
I find the idea of two men getting it on far less than pleasing in aesthetic sense, but there are things that are more important than aesthetics such as basic human rights. So as long as they aren't doing it in front of me I don't care. There's no logical reason to treat them as lesser.