I have no interest in sex other than for work and have been like that for a long, long time.
I'm only in my mid 30's too.
I think my hormones have bought a one way ticket overseas. Maybe another reason is all the bad sex I've had over my life. The only good bits were in my teens and very early 20's.
More times than not I think I'll only have intimate encounters again if I'm getting paid. Otherwise, what's the point?