I can only relate my own experiences. I am positive diagnosed DID. Especially lately, with my SO, some of the Alt's have been friskier than others, but it's been in a co-concious way. It is very rare for an Alt to front during sex, but I have noticed that during some very intense orgasms, that different Alt's will front during the process. It's only during a more intense orgasm, and I find that the Alt that quickly fronts uses it as a means to expel extra energies stored up. For instance, my protector Alt, will front during the orgasm and experience it in a very dark and intense way...almost purging negative and harmful energies through the physical act and the exhaustive toll it can take.
I have been co-concious for all of these (at least I HOPE I have), and I have to say that when I finally get to a relaxed state I have found it has helped calm some of the inner turmoil. I have had a few experiences where, during orgasm, I am aware of more than one Alt switching out during it all, to experience the benefits of the physical (and mental and emotional) release.
To be completely honest, and not to sound crude, but sex is a big part of my safety plan. I find it helps my SO and I stay close, even during high stress times, it's good physical exercise, and it helps purge some of the clutter that is in the brain, that I may not be aware of. I find in times of higher stress, when I am alone, sometimes masturbation also helps calm the mind. I'll also add as a quick aside, that I am also diagnosed BPD, and sexual dysfunctions can be triggered, so it also helps keep my drive in check when I am looking for an "unhealthy outlet" (ie in a high state of trigger heading towards little control over switches and fronting).
One more thing, and this is tantamount, I have an AMAZING SO, and we discuss everything openly and very honestly, this helps keep us both in the know for desires, ideals, fears and coming up with plans to keep things flowing in a generally fluid and safe manner. We have discussed the issues of sex with Alters, or if an Alt presents wanting physicality...these are all important discussions to have with someone with DID. Better to know what everyone wants and expects than to fumble through it. And, DID/Singleton partnerships are just that, 2 people, both with needs and wants. it may help to really listen to the other persons thoughts as well.
I am not condoning going out and getting relief with anything someone can find lol, but it is definitely something that, with my exceptional SO, has become part of a healthy routine that is benefiting my whole system.

Hope that helps. I'm a bit fuzzy.
P.S. I also think, for those with DID, and having Alt's with a higher sex drive, or fantasy system in place, that safely engaging in communication about sexual wants/desires vs. safety and responsibility can lead to a better understanding of what all parties are looking for. Again, I can't stress enough Open and Honest communication.