Arakure
Honoka ni Purple
Uwasa no Midori-kun!!
Zettai Heiwa Daisakusen

There's also some in Hot Gimmick

though you don't know it from the start.

The same possibility exists in Honey Hunt (same mangaka), but it hasn't been shown as such just yet.

The hints are there, though, so it's likely coming.

Smut:
Akutou Danshi Collection
Forbidden Phrase
Kindan Princess
Love Monster

Oh, and they are technically married in Please Teacher, but that doesn't mean it's not awkward or that they know what they're doing. =)

If you've ever thought or said "Nice Guys finish last" and really meant it, then you should probably read this LJ post by DivaLion. It's incredibly insightful whether you're male or female.

From a bumper sticker I like:
"If you're gonna ride my ass, at least pull my hair."

blood alone

Live, love, laugh <3

Hana Kimi
So Bad (same person that did Hot Gimmick)
Absolute Boyfriend
Marmalade Boy
From Far Away
Ai Hime - Ai to Himegoto and Kiss/Hug (same artist)
Model if you are counting vampire stuff.

Blood Alone, as advised earlier
Kiss x Sis, also as advised before
Chokkoto Sister
Toshiue no Hito, this one is done by someone who used to write hentai titles, so you should know what to expect.