The laws already establish that teens under a certain age cannot legally consent to sexual activities (at least with someone above the legal age) or sign contracts, so the rules on that would be unchanged. Likewise, while the President of a company can already legally marry one of his interns, that doesn't necessarily contradict corporate rules governing fraternisation--it just means that one or both of them would have to quit for the marriage to work.

That's the approach I'm taking to consent in the case of gay marriage and polygamy. A man doesn't have the right to marry 5 teenage girls or even 1 teenage boy. I see no reason why he shouldn't be able to marry 5 grown women or 1 grown man IF they legally consent.