I used to read extensively in X-Files, but I was a definate MSR shipper. I did occasionally dabble in some slash and threesomes. This was my first exposure to slash. If you're still interested I can point you at some of my old favorites:

MustangSally - great voice, dark angsty sex and suprising humor.
http://fluky.gossamer.org/author/10861-1.html

Lisby - threesomes, MSR, Mulder/Skinner. She's got an unfinished non-com somewhere where Mulder is taken for a sex slave, but I've long since lost it. But that kind of tells you how far she'll go.
http://fluky.gossamer.org/author/11246-1.html

Wintersong - Start with "Communication" and "The Games We Play". The Lost series is an absolutely fascinating look at their partnership but it never gets quite as far as sex. Read it anyway.
http://fluky.gossamer.org/author/14834-1.html

My very favorite X-Files writer is Karen Rasch. She's not always angsty, but damn her characterizations were dead on, and the sex was hot, hot, hot. Sometimes she even gets kinky. Oh, and she writes good plot, too. I'd start with "No Greater Love", which should have been filmed, dammit. Then dive into the "Words" series.
http://fluky.gossamer.org/author/10046-1.html

And here's a kinky M/S/K thingy with non-con and such. *eg*
"Making his Marks, by Satina
http://fluky.gossamer.org/cgi-bin/read.cgi?ln/MakingHisMarks.Satina

Oh, and you might also like Amperage.
http://fluky.gossamer.org/author/11985-1.html

If you manage to get through all of these and still want more let me know. Like I said, I used to read a lot of X-Files.

Love ya,
t