Mike just had so many personal insecurities about himself. Dude never got to learn to love his physical appearance because Joe was teasing him as a kid about his nose and then dude gets older only have skin issues like acne, lupus, and vitiligo. Add in to the fact that he got that bald patch in his scalp because of that Pepsi commercial incident.
Dude was probably intimidated of sex too. Performing at strip clubs and witnessing your brothers smash groupies all up in the hotel room at such a young age probably wasn't good for his maturation.
I personal can't blame him for not wanting to carry on his genes when he don't know if his skin issues will be hereditary.