I don't think this is quite a duplicate of this related question, or this one although it is very close to both. I am as a GM and a player, by my reserved nature as a person, somewhat reticent to engage in sex-related themes in a game at all, period.
That said, I recognize even from my own experience that some situations can feel a little more fraught than others: Usually when my or someone else's Significant Other was directly involved and especially when the "someone else" was there, too. From what I've read of interviews on the subject of sex scenes with married actors, apparently that's a lot more fraught than it appears on screen, too, so we are not alone.
What I personally do in these situations (if I am the GM) is dial the entire campaign back to the level of comfort I have with that individual player. Generally speaking, that is not Puritan/Disney level, but does involve a lot of Fades To Black as described in several answers of the related question. This is best done early in a game, though, since even though it is egalitarian and "fair" it may result in a major part of a PC's personality becoming useless.
Two other things you can do include:
1) Talking to the player, explaining the issue, and promising to at least try to do a little better each time. Like most things, practice and repetition will desensitize you. I have done that in related situations (I often use convention games or other one-shots as experimental venues, so if something blows up in my face, the damage is limited) and it has helped.
2) Delegating a certain class of bit-NPCs to other players, if the nature of the scene allows it (i.e., if the delegate player's character is not also on-scene.) Be warned, I have not actually done this, though I've entertained the idea for other situations, so I can't vouch for how well it will work.
Finally, remember that in general no one should be playing a character whose nature makes another player uncomfortable. It would probably not be controversial to suggest that if a promiscuous PC insisted on playing out that promiscuity with other PCs as the targets, and if one or more of those other target PCs were uncomfortable with this, then the player of the promiscuous character ought to stop it. Remember though that the GM is a player, too. You've correctly determined that you have an expanded set of responsibilities due to your role as the GM. But that doesn't mean that the other players have no responsibilities toward you.