I think this decision depends on whether your being with either of them overlapped. I would say that as long as your relationship with the friend was prior to you being with your current boyfriend, there should be no problem relating the fact that you guys had sex to your boyfriend. Chances are that if he already does know about your past relationship, your boyfriend likely assumes you and your past flame were intimate at some time. Even if you had a romantic encounter with this other person not too long ago, there is nothing wrong with living up the passion with and old boyfriend, as long as you are not in a new relationship, that is. Although this person is your boyfriend’s pal, I think telling him would only build trust, while not telling him might arouse suspicion on his part if he finds out later from his friend. Why would you consider your former relationship worth hiding from him, your boyfriend might ponder. No reason to be secretive, even if you think it would be weird for your boyfriend to find out (which it may likely be, just as you would likely find it odd to think about your boyfriend having dated your best friend in the past). Fact, remains that you had a previous relationship in which your current boyfriend had no involvement whatsoever and that past relationship is now over and you were ready to move on. If your boyfriend voices concern about any still existing feelings for this other person once he finds out that the two of your were intimate just months before meeting him, you can always tell him that if you would have truly wanted to be with your ex again, you would have pursued a relationship/new start at that time. But being with him again, just made you realize that the two of you in fact had little in common and no future together. Then highlight some attributes that you love/admire about him and lay it on thick, if his ego is truly bruised about this matter (which it should not be, since you did not cheat on him).

Consider this:

Do you really want to spend time with someone that you CAN'T tell everything to? That you are afraid will find out something about you and leave you? That wouldn't accept you in spite of everything you may have done?

Why people even consider sleeping next to someone they don't trust 100%, I'll never understand.

Fortunately I'm with a woman who knows everything about me and I know everything about her. We love each other absolutely, no matter what. I can say this because we knew each other for 13 years before we even tried dating once. We not only know all about each other, we were there when it happened! :)

If this man of yours isn't interested in knowing everything about you and able to appreciate it he needs to find someone else and so do you. Unless you're just using him until Mr. Right comes along. Then lie to him like you would any other guy you didn't have a vested interest in. After all it won't matter after a while anyway, right?

It's up to you the sort of foundation you choose for a long term relationship but if I were the man I'd not only want to know, I'd want YOU to know it all about me! But the bottom line is if you wouldn't want to know everything about him if the situation was reversed then you can't be expected to treat him any differently - BUT you should make sure he KNOWS NOW that that's a little rule you have so years later he doesn't find out and leave you for being dishonest...

I actually was on the other end of this, and found out that my boyfriend had a previous sexual relationship with a mutual friend of ours. I have been friends with this girl since we were kids, so it was upsetting that he hadn't said anything. It made me feel unimportant. However, I think "we had a romantic relationship" is enough information, and he knows that. If he asks if you slept together, and you want to be honest, tell him. If you don't want to tell him about it, just say flat-out that what's in the past is in the past.

While I do believe all relationships are better with openness and honesty, it's not necessary to quiz each other about past sexual encounters. A lot of people wouldn't even want to know the details of their partner's sexual past, as it could just be hurtful.

I think you've done enough by telling him that you had a relationship with one of his friends.

If you didn't tell him before you got involved then never and in the end this relationship won't last anyway. It will eventually be found out by a third party.