I agree with Nana, don't make it all about him finishing. Make it equally about you and your satisfaction too, which hopefully will make you relax more. And don't beat yourself up about this, truly.

Sometimes this will happen with my boyfriend and I and it's very normal.

In this scenario I would talk about it very clearly and bluntly. Next time it happens, calmly chat with him why he thinks he can't finish. Is there something he's not expressing? It's his responsibility too to be open and communicative about what works for him in bed when you have a string of incidents like this. I'd use a tone that won't make them feel as self conscious he probably already does.

If you are naturally concerned about him having a good time too, and asking him about it, then he shouldn't leave you in the dark confused. I will say though again not to be too hard on yourself, as sex takes awhile to figure out between two different people.

This is probably one of the strangest things I've ever heard. Do you meet guys at a place where there is a high amount of sexual dysfunction? Do you talk the whole time or are you particularly loud and distracting during the act? Do the guys drink a lot beforehand? I'm trying to think of reasons, most guys can orgasm at the drop of a hat, especially when having sex with a woman. Is there something that is different about you that is causing this problem?

Okay, I don't want to get too personal, but what are you doing while he is going at it for 1 1/2 hours?

Because I'd be saying "Ow, ow, ow get off of me!"

That said, it's not all your fault. It can't be, because there are two people involved.

Try not having actual intercourse until he's just about ready to explode. Starting intercourse before full physical arousal occurs is one of the main reasons women don't orgasm and I see no reason that can't also be true for men.

The main thing to do is relax. Sex is supposed to be fun. If it's fun without him finishing, then forget about that as your goal. If it's not fun, then keep trying new things until it is.

Just stop blaming yourself, okay? You'll sort it out.

Anon, it is not your responsibility to make sure a man finishes. And if any man implies that it is, he's an asshole. Sex is a team effort.

The only thing I could imagine that would prevent his orgasm that could be related to you is if it's clear to him that you're not enjoying yourself. So if you're lying there like a dead fish, texting and checking your watch, that's a problem. However, if this was the case, and three boyfriends were experiencing this with you, they would have said something about it. You wouldn't be left wondering what you're doing wrong.

Your only responsbility is to be communicate honestly about the experience, and enjoy yourself. Tell him what feels good to you, and ask him to share the same with you. Beyond that, it could be anything. Maybe this younger generation of guys grew up on porn and have a hard time getting off on regular sex. Which is their problem, not yours.
You also didn't mention whether you orgasm with them. Maybe they're holding out waiting for you. You're not obligated to finish each time, and some men think women have six orgasms because that's what happens in porn.

Is this something that happens all the time, or just once in a while? Because it is completely normal for this to happen here and there. Especially if he is stressed, or has been drinking, or tired, or any other number of reasons.

With sex, as with life, the most important thing to focus on is enjoying the journey, not the destination.

What are you using for birth control? I ask because some guys experience discomfort with IUD's or NuvaRings, and certain types of condoms are not as comfortable for men as others.

Is there typically drinking involved?

And when you say your past three boyfriends... have all three just had a random occurence where they just couldn't come? Or was it consistently that they couldn't the majority of the times you had sex with them?

Are you the type to get really rough with a guy and take charge? Do you lay there and just chill? Do you stare into his eyes the entire time? THere are certain things that could be going on that just make him not able to climax that aren't necessarily your fault, but things that you just could be doing differently.

The men I've known who had trouble getting off all had one thing in common: They watched a lot of porn and jerked off a lot.

No, I'm not saying porn is evil, no I don't think you grow hair on your palms from jerking off.

But this is a well-documented problem, and A LOT of men watch too much porn because it's so readily available. And I would bet my bottom dollar that's the culprit. A couple of things happen: First of all, when they're jerking off, they might be making too tight of a fist, meaning no matter how tight your vagina is, it will never be as tight as a closed fist (especially since we expand when we're aroused). Also, they get used to certain triggers that get them off, and soon nothing can get them off EXCEPT the porn they're watching. And these men--because they're well practiced--are very proud of being able to go forever and ever and ever.

But then they can't get off.

So I would say--to make it sexy--tell them you want to do a little tease and denial game with them. They aren't allowed to watch porn or masturbate for a week NO MATTER WHAT. And during that week, you're going to flirt, your going to subtly touch them, you're going to be suggestive, but you're not going to have sex/dry hump/hand job/blow job/nothing.

And at the end of the week you're going to have sex and he's going to cum.

And then you're going to (lightly broach the subject) of porn use--if that is what this is.

And also another thing to consider is that you're getting worried that they can't get off, so as the sex goes on, instead of getting more intense, it's just getting more tense and stressed out and bored and "Oh my god, would you just get off already! What's WRONG with meeeeee???" Which isn't all that sexy.

I would guess it's a combination of these. So if your guys are watching a lot of porn, and you're thinking, "Well, hey, every guy watches porn" know that not every guy watches porn every day--and it might be the problem.

Seriously--even those awful websites and gurus about "pickup artists" write about the dangers of too much porn use.

When a young guy can't get off when an attractive woman's underneath him, it's almost always too much porn and too much jerking off.