Does my S/o like men?

Recently i went theough my so email and saw he had been responding to sex ads on craigslist the person who responded ask him if he cared if it was a man or women and he responded with "tbh i dont care im just trying to nut" im upset that his doing this but even more upset he said that, were in different cities right now so ik its hard only having sex on the weekends for him but thats not an excuse, we got past him doing this but not past the gay part when i bring it up he blows it off, should i do the same of does it sound serious, i need advice?