Just agree that it happened
Picture this. You're roleplaying a young gnome who has fallen in love with a seasoned, world-wise night elf warrior. You've decided that your character is in love. And, admittedly, perhaps an equal part lust. That's okay -- it's not a sin to decide that an excitable, curious gnome would feel that way.
In a quick out of character conversation, you and the night elf's roleplayer decide that the warrior is going to take advantage of the gnome's interest. After all, the night elf is a lonely character, and would probably indulge in an evening of passion. But neither you nor the other player are looking to engage in the visceral description of roleplaying that night of passion.
The two of you can simply decide that the night happened, and leave it that. Pick up your roleplay immediately after the night has taken place, and start roleplaying through the emotions and interactions each character would have after the fact.
You don't need to dwell in the details of the sex. Despite what the internet-at-large might tell you, the general mechanisms of person-on-person intercourse has been relatively the same for millennia. If the point of the story isn't the exact permutations of how the two characters made out, then there's not really any point in delving into the details.
Of course, what if the point of your roleplay story does involve the details?
Keep the descriptions very high level
Let's take another scenario. Say you're roleplaying a battle-hardened paladin who has finally found someone to fill the void in his life after the death of his long-lost wife. You've been exploring the growth of the new relationship with an open-minded, wise mage. After weeks or months of playing together, the two players agree that the two characters would finally consummate the relationship.
But your paladin isn't as emotionally prepared for the big moment as he might have thought. You want to have the character suffer impotency when trying to do the intimate deed. The detail becomes important to your roleplay, since the mage would certainly have some kind of meaningful reaction to the event. There's some important discussion at that point, and you don't want to lose the opportunity.
Instead of trying to be detail-oriented through the whole sexual encounter, stick to high level descriptions. You want to pick up roleplay at that exact moment that the details matter. You can discuss this situation at a high level. "As we're both naked, I try to do the deed, and you find out I'm unable."
You don't have to be too descriptive with the events. Euphemisms and general descriptions are the key here. Say things like "When we go into the tent, my character is gentle and careful to take his time. Anything notable as we proceed? No? Okay, then as we're at the big point of performance, my character is unable to remain ready."
And then go from there.
Keep in mind what you're trying to get out of the roleplay. If you're after a thrill, then sure, the detailed, visceral descriptions are probably what you're after. But if you're trying to explore deeper, more intricate emotions, then stick to the pivotal actions that will set up that roleplay.
Stay in communication with the player
A lot of players will be extremely uncomfortable with anything that hints at erotic roleplay. And, really, who can blame them? There's a lot of baggage and squick tied up in the whole idea. You would be doing both yourself and the other player a huge disservice not to talk everything out on an out of character basis.
It can be an uncomfortable situation to ask another player whether or not you really need to roleplay out the intimate relationships. But that just proves why you need to keep the lines of communication going at an out of character level. If you just roll over into playing the sex out, then the other person be even more uncomfortable than that on a greater order of magnitude.
If both players decide that they want to let their characters become intimate, then you absolutely have to agree on how you're going to describe that activity. You can keep the description at a high level, simply agree that it took place without engaging in any description, or pursue any other method you might prefer.
But, then, you need to not make any assumptions about what happens in the future. (Boy, how many dates have gone poorly because of people making this error in real life?) Just because you've engaged in intimate roleplay once doesn't mean that it's going to happen again. Heck, if you're not after the visceral details, you may just decide that there's no point in further roleplaying the intimacy. Just kind of agree that the characters are active, inactive, or intermittent; then, move on with your roleplay.
You don't need to advertise
The last point I want to make about this subject is that you probably don't have any reason to advertise your character's intimate relationship. Oh, I'm sure there's probably a dozen characters who brag about exploits, or for whom this kind of thing is pivotal gossip.
But no matter what happens, there's no reason to go spreading about how you handled the roleplay out of character. Erotic roleplay -- and even the idea of characters having sex -- is going to be a huge point of contention among groups. It's best just to leave the subject as a non-issue.