I'll add that we can see two straight actors, female and male, playing characters who are supposed to be romantically and/or sexually involved, and we don't buy it. How often do we hear, "They had no chemistry. I didn't believe they were in love or that they were attracted to each other"? This may also be true of gay actors playing gay roles or straight actors playing gay roles and trans actors playing straight or gay or trans roles and . . . well, every permutation that's possible. We might sometimes say, "I didn't find that actor believable as gay" when it sometimes might be a matter of lack of chemistry with another actor. Other times it may be that the actor just cannot make us believe that he's attracted to people of the same sex under any circumstances.
So there are many variables. Straight actors certainly can play gay roles successfully, and we've seen it, but there's also nothing wrong with saying, "I couldn't buy him as gay" or "I didn't believe he was in love with or wanted to have sex with the person he was supposed to be involved with."
I'm also sure that we've all experienced this in real life as well. A couple that seems mismatched somehow. Or someone who doesn't "seem gay" but who is, etc.