My wife has fucked me with her dildos and its hot! The best feeling is her fucking and sucking me at the same time! I love it! I am thinking about buying her a strap on to fuck me! You should do it for your husband. Does he want a real cock in his ass?
Everyone has their fetishes. If it makes you uncomfortable, just tell him politely. I'm sure there's is some time of compromise you two could work out.
Your husband can only be gay if he deserves to have sex with another man. If he is just asking you to use a dildo on him, then it's not gay. After all, the prostate is basically a man's G-Spot.