He didn't "wait until the IPO" according to people who knew, he waited for her to graduate. The timing of the IPO is very hard to predict given it requires sign off by the SEC, graduation on the other hand, pretty easy.

I don't doubt for a minute that they have some sort of agreement in place in the event of a separation. While most might think such an agreement would benefit Mark, the real beneficiary are Facebook shareholders if splitting up required selling shares to split proceeds. That could really move the stock unreasonably.

However, as I know a number of people who are (some of whom merely were :-) very very wealthy, they share a common difficulty in dating post wealth. Not that every potential spouse is a "gold digger" but the people who are inclined to be one come out of the wood work apparently. It can be a very sad thing, being lonely and knowing how that makes you a target. So having someone you know (and knew) when you were just a crazy dreamer, and they still wanted to be with you, is a very priceless thing.