If (in some kinda weird recursive time-travelly way) it could be shown that the relationship would last the test of time, be a happy one, and foster balanced children (or whatever you wish from the future of the partnership) - despite one partner having cheated or had an affair (or even several) which was (/were) never disclosed - then I wouldn't want to know.
BUT - we don't know these things. It's all hypothetical - and aye, if someone's cheated, it's usually due to something underlying, and I'd want to know so that I could have the opportunity to get out before something else slips.