Try losing some weight!
lol.. I'm just kidding, that was a joke.. lol.. i'm sorry, it's late at night here in NY, and i'm about to pass out in a while and call it a night..
Seriously.. it could be a number of things.. here are the most common..
1. His penis can't support you. No, you're not fat; maybe you are, but that's not why it can't support you. Even for women who weigh 100lbs, combined with movement and strange angles, the penis can actually be experiencing more pain than pleasure, which will cause it to get soft. (Solution: try having sex sideways while you are both lying down on your side, and he is on your back).
2. You take TOO long from the time you initiate, to the time penetration happens. The male body is very practical, and that being said, efficient. Once turned on, the penis will remain hard for a very limited amount of time before quickly turning soft again. It needs constant stimulation, or at very least, frequent! If you initiate, and then take more than 45 seconds in interuptions (example: it's not going in, outch wait, go on top, no I want to go on top, etc) it's all over. (Solution: keep interuptions under 10 seconds! If you think you're going to have trouble having him go in, relax, let him go down on you first, and then start sex, not in the opposite order!)
3. Maybe you're just not sexy. I'm sure you're very pretty, and i'm sure you're very nice, but those kind of qualities will get you a second date, NOT an errection. Remember, guys like teasing, lots and lots of teasing. Make playful faces, (faces like you know you're sexy, and you know he can't resist you, you know he wants you, you know you're driving him crazy). It's not a bad idea to come up with something witty-yet-sexy to say that goes along with that face. Secondly, dress sexy. Unless the jeans are so low-cut that you start a boy parade behind you, they're NOT sexy. Reconsider what you wear as regular undies, and what you wear as sex undies, the two are very different; (for most women anyway).
4. It could be HIS fault. Make sure he has NO alcohol before sex. Tell him to drink something acidic like Orange Juice. You want his blood Ph to be slightly acidic, and his blood pressure to be slightly low. A diet high in fatty foods can cause problems with erections as blood has a harder time filling the spaces between the muscles in the penis to make them swell. Lastly, plenty of water! Water, not too much sugar, stay away from fats, no alcohol, and drink something acidic like Orange Juice.
Well, hope this helps..
Best,
If you can't stop the Wind, then you can't stop the Storm.