Just like in the rest of society, it's easy to get people to focus on the relationship side of homosexuality. Accepting that non-heterosexuals are real people that do more than have sex and relationships with people of the same sex is a bit more difficult for some. It's hard for people to see that we are just as capable of headlining an action movie as anyone else, technically speaking.
Whether the output of Hollywood is limited by a stereotypical America accustomed to white, heterosexual, cis-gendered dudes and afraid of change or whether it's done by executives who buy into that stereotype when making investment decisions is vague enough that one side--when accused--will always point the finger at the other.
Ultimately, someone thinks a romantic comedy with a gay/bisexual/other lead has a niche audience with an okay return on investment. Those same folks think an action film with a gay/bisexual/other lead doesn't appeal to either gay or action audiences and ends up being a shaky bet. It does boil down to investor confidence. I'm certain there are people pitching these movies, but they haven't sold too many to either investors or audiences.
The deck is stacked against non-heterosexual leads in movies that aren't about how they're not heterosexuals. Look at Commando. For all we know, John Matrix is a bisexual. If Matrix kisses a female, no questions are asked. If he does nothing romantic, he's still assumed to be straight because characters in movies are assumed to be straight until proved queer. The creators can come out later and say he's bisexual (hello Dumbledore) but the narrative still don't deliver on that. If he kisses a dude at the end, then folks suddenly interpret that as a contrived, artificial message shoehorned into the film.
Despite the completely stupid, impossible things happening in Commando, people wouldn't accept non-standard sexuality on the part of Matrix because it isn't compatible with the action movie "reality" they're accustomed to. Violence, stoicism, and strength are coded as masculine behaviors which are just some of the virtues in action films and they're deemed incompatible with homosexuality. In fact the contradictions of gay men as "non-male males" fuels the comedies they're in. The atypicality of other LGBT characters does the same.
In the eyes of those stereotypical Americans, you can't put "non-standard" characters in certain roles. That's why you have to create some contrived situation to explain a black Johnny Storm even though biracial families are a real thing that exist. It's why when women fight men in movies, it's usually either the woman catching the dude by surprise by hitting him over the head with something or a woman who has an excuse.
For example, the female in Live Free or Die Hard is Asian, and therefore a kung-fu master(!?) and The Black Widow is a full time super-spy who has trained her entire life at the exclusion of anything else about her. Notice how Black Widow doesn't seem to have much of a personal life. The assumption is that a woman beating a man in a fight is so "unrealistic" that she needs to spend her entire lifetime training to do it. I don't think I need to go into detail about how messed up it is that a random Asian female is allowed to have skills equivalent to The Black Widow because of her parentage. That is a whole other sack of bullshit.
 That might change in Captain America: The Winter Soldier, some four years after her introduction in 2010's Iron Man 2.