Interesting how threatened women, even apparently bisexual women, are by bisexuality in men. It's a total no-no. Why is it such an insult for a straight man to be thought bi or just potentially bi when he is not? Who cares? Gay men are taken for straight all the time. In fact, 'straight acting' is a kind of compliment or pat on the back. If you think it's insulting to be thought gay, that's probably because you think gay is an insult.
I'm not threatened. I've even detailed how I would be open to having a 3-way with another man if my guy told me he was into it. He's just not though.
If I ever "defend" his straightness it's not out of insecurity, it's out of respect for him as a person. He wouldn't have sex with a man just because of "a few beers" just like @Tattooed Goddess says about her husband.
I wouldn't judge him if he did, it just gets under my skin (and his) when gay/bi men try to make that argument. It's rude and inaccurate.