In today's world being called homophobic, racist, fatphobic, nazi, xenophobic, etc. is a badge of honor as, quite often, people throwing those words at you don't even know the meaning behind them.
Not liking girls hooking up fits into the meaning of homophobic term. A term that is used to describe someone who dislike and/or have prejudice against gay people and actions. A term that exists like many others, becasue people are, by design, lazy and do not like to use too much energy, including talking or writting if it ain't necessary.
Yet, in order to have a reaction, like the dislike of lesbians, we need the action that caused it, the reason.
Only by asking the man who is homophobic why he is homophobic we can learn it or work it out, because he may doesn't know fully himself. If not, we can only try the guessing game.
We're slaves to our emotions, and the reason may be basically dislikining lesbians, due to not having option to participate with them. Not liking girls to hook up may be feed by man's fear of girls growing on eachother so stong they might forget about him.