Meta ask—Ok now I’m curious, how do people who want to fuck demons do so?
And thank you for rping with me it’s so fun to do
Thank you for contacting the Department of --- oh wait. Okay meta time XD
So, "demons" in the DOMA setting means something slightly different than it would to the average person. It isn't demons as in the Christian idea of evil spirits or devils or minions of Satan but more "demons" in the..."anime translation"(?) sense. Basically, any extremely harmful, evil, or corruptive monster is called a demon. Vampires are demons, devils are demons, eldritch horrors are demons, etc. Therefore, how people fuck demons depends on the type of demon.
As you had asked about more classical "devil" demons I'll just focus there though.
Partially, it does depend on the type of creature they want to fuck. Cubi are probably the easiest as Cubi can be found at nightclubs, raves, college campuses, and really anywhere a lot of horny people are together. A random Cubi is more likely to visit a person's dreams than have physical relations with them but Cubi react to people's desires, so if someone wants it enough it will eventually happen. (Of course, this will probably lead to them becoming addicted and likely turning into a Cubi themselves after a time.)
For things like Imps it is a bit more difficult. Imps may be demons but they are allowed within society in a confined way - mostly through Instant Imp with a few mages or researchers having licenses for bound imps too. Someone could try to seduce an Instant Imp imp when ordering for a quicky and the imp might take them up on it (assuming he doesn't think it's a trap). A mage or researcher probably wouldn't risk their license. Best bet here for people would be to find wild imps that weren't captured after the Great Incursion, they do still wander around in little gangs or by themselves.
The most reliable, though still illegal, way for people to fuck demons is to either do a summoning themselves or to find an occultist, mage, or witch willing to do that sort of summoning. This has definitely turned bad for some people (thus DOMA's warning) but I would say the majority of summonings do work and go fine with DOMA none the wiser.
Of course, there is the possibility of a pact devil just showing up without a summoning if the individual makes their desires known. Typically, pact devils enjoy the slow corruption of someone, the temptation, but an easy soul is an easy soul.
Finally, if someone really just wants to spend their life being used they could enter the quarantine zones. The ruins of Washington D.C would be the best bet as it was consumed by Hellfire towards the start of the Great Incursion and lots of more devilish demons are there. It is physically dangerous for sure there are bone-nashers, hellhounds, and all sorts of "just eat you" sorts of demons there. But if someone could make it to the area protected by one of the soul-collecting or corruptive demons they would probably be safe and taken in as a servant with the possibility of being fucked by their master or a demonic servant or ally of their master.
Oh! I nearly forgot there is also the possibility of cambions! The children of a Cubi or pact-devil and a human mother (usually a witch or mage). While these are rare and they face heavy discrimination, they are technically allowed in the Federal States because they don't inherently corrupt. They may be able to cast corruptive magic and often do embrace their demonic side but there are resisters out there. Cambions are basically all the perks of devilish demons with none of the downsides (unless they are evil).
So, yeah, there are lots of different ways to go about it, it is just dangerous.
And thank you so much for the asks! It was a lot of fun having to respond to an off the wall situation/question rather than a general lore question. I'm definitely a slut for asks though, so if you or anyone reading this do have questions about the lore or any of the monster lovers (or want to do some RP) never be afraid to ask!