If a guy see's another guy with a hot girl he may be jealous, if a guy see's 2 lesbians he hopes to join in.
If they are lesbians they obviously don't want a dude jumping in. Different story if they are bi-sexual. And even bi-sexual girls are picky about the guy they may or may not be willing share with each other!
It could be if 2 women are lovers they may not want to fuck each other with a guy present unless they get a kick out of cock teasing the pre-cum out of you!
Either the poster of the comments is confused or a majority are when it comes to lesbian and bi-sexual!
That makes me laugh with professional produced porn their not lesbians just because you have two women, when in the next scene one or both are getting gang banged. Or they have some gay looking nut sack appear to do both!
The point being two women does not make it lesbian they could (more then likely) be bi-sexual. Lesbian is wanting to fuck ONLY women, and they have absolutely no interest in wanting to fuck a MALE!