Its sexual assault because it was not something you simply agreed to right when asked, you likely didn't fully understand how anything works or what was really being asked of you so you could not make a valid choice on something being right or wrong, and also you didn't for lack of a better term explore this type of thing on your own because you thought it could have been something you might like. If its starting to bother you now it could be because you had it on the back burner while you got on with life. Sorry if this isn't helpful or what you wanted to hear, just my opinion, and also as a rape survivor.