If you really have feelings for someone and you spend a lot of time together in the first 3-4 days and it feels right, what does it matter? My girlfriend of 6 years we had sex on the 3rd day and we've lived together ever since, but it's kind of an unusual case I guess.
If you aren't putting any limitations on yourself as far as having to be in a relationship to have sex, I don't think it really matters how long you wait...The only girls I actually think of as "sluts" are ones who's self-esteem is so low that they go around and almost pathologically start sleeping with a bunch of different guys...especially if it's a bunch of guys who are all in the same circle of friends and she's leading them all on and creating problems....That's not cool...
also, it's none of your exes business who you decide to date, but if you guys are still hanging out in the same circle of people and you're sneaking around so he won't find out, that IS a recipe for disaster. Unless you genuinely have feelings for one of your mutual friends, I'd avoid fooling around with your mutual friends. Of course, guys are notorious for doing this kind of shit, partly because it's generally the easiest way to get a quick lay....but it's bad morally, not cool...