Husband is 100% straight, but strong enough in his sexuality to have 'considered' whether a bi experience would interest him. It doesn't. But, he loves anal anything. He's got no problem with worrying over being gay because of it. If it feels good, it feels good, end of story. Now, I do know men who wouldn't even consider it because of the 'idea' of it, and have likely never tried. Sex is so mental though those men could never enjoy it if it did feel good because of the fear.
So, somehow gently bring the subject up, or as the other poster said, gradually move that direction and guage his response. He'll either ask what the hell you think you're doing, or moan for more.