What is it like to be a gay man in a heterosexual marriage?
I've not done it, but I know people who have. Some make it work, others don't. It depends on how well they trust each other, and what they want out of a marriage, and do they communicate with each other.
None of the gay people I know who got married did it deliberately to lie, or with an intent to hide. All of them were either denying that they were gay or trying to not be gay or weren't able to admit to themselves that they were gay. That's how denial works. It's not a lie - lies are deliberate and on purpose. Denial of being gay is not the same as lying about it. That's something that I think straight people don't understand, because they've never had to ask themselves if they are different from everybody else.
I don't think it's fair to call them cowards either. It's fear based or anxiety based, but that's not the same as cowardice. The fear of retaliation is real and reasonable. That doesn't make a person a coward. Wanting not to be condemned is not cowardice, it's reasonable. The real issue is that there are still straight people who judge and condemn LGBT people, and that includes people who call this cowardice. Closeted LGBT people need to hear from straight people that it's OK for them to be gay. It doesn't matter if they hear it from other gay people, they have to hear it from straight people. Calling them cowards doesn't do it.
For centuries gay and lesbian people got married. But it wasn't known and so it wasn't seen as some sort of "phenomenon."