You may all be surprised at how many ‘straight’ men there are, who are also married to women, who have profiles on gay sites and apps looking for sex with men to varying degrees, there are those of an older generation who were never able to come out, so are married with families and have for decades sought out other men, there are some younger men who are also married who come from backgrounds where religion does not allow homosexuality to flourish and there are those who were raised in hostile environments, who also pursued relationships with women.
The world is a harsh place at times, however there is now, at least in the western world, NO excuse for ruining a woman’s life to cover for being gay. It’s sweet of you OP to be worried for him, and of course be his friend throughout whatever happens next if you want that, it’s your life, but please also remember, he is using you and wasting both your lives, you could both have partners who love every bit of you.
It’s already been said, but NO straight man would join any gay dating space or gay website for chat. All the straight men I know who are comfortable with their own sexuality will openly love and hug their gay mates, be there through everything together, but their focus for relationships of a romantic nature are always women. Gay people can live freely now and can escape homophobia from families and communities, find their people and live a ‘normal’ life together, all the boring stuff, keeping house, shopping, just living. There is no reason to lie and also drag someone else along for the ride. By all means do it amicably, but please do start looking out for yourself and leave, I’m sure you would if it was women he was seeking out. He’s gay and knows it, is getting defensive when you question him about it, which is a clear sign of covering up a lie, don’t live the rest of your life with someone who’s not in awe of everything about you.