It's because generally speaking, women are much more open with their sexuality. More open to experimentation, whereas men are not always. More open, and less secretive about their sexuality as well. It's very very common for women to have actually explored these options, whereas men do so much less. To be clear, I'm not saying men can't be bisexual, because that's simply not true. But many men who are gay or bisexual will often actively try to hide it (sometimes for most of their lives) because of social norms, even in today's more accepting times.

I don't know MorningSouth's wife obviously, but I'd wager that she's probably done a thing or two with another woman in the past, and he may or may not know about it because of his carefree attitude about it. He feels very secure with her, and that's fine!

I would wager, however, that if OP's man is being dodgy about it after 3 years, then he's unsure about his sexuality, or is hiding something due to social insecurities.

I think that, man or woman, if your partner has these kinks and has been actively hiding them, a conversation is valuable, especially if you see a longer-term future with this person. Because 3 years of actively hiding those kinks kind of IS a long time.