I think I misremembered parts of this one. Spock was in love / obssessed with Kirk. Kirk was kind of using him for sex when there was no one else but he was also agreeing to dates and then standing him up. Spock got it into his head to knock kirk up to bind him to him. Oh yeah and Spock starts to lose his mind.

I don't care as long a you're mine by Ivycross.
http://ksarchive.com/viewstory.php?sid=4903&warning=1
Warnings: Non-con, Mpreg, dub-con, dom/sub.