Well, you know they're soft... why not?

Sarah leans down to your cock and begins licking it madly. After moments, your rod is literally dripping with spit. "Lube," she says casually, catching some of her own spit in the valley between her breasts.

"You're in a mood tonight, aren't you," you moan. She smiles and nods, lying back down. She pushes her tits together, her pale flesh forming a very inviting valley. You set your dick between them and enjoy the feeling for a moment. You close your eyes as you feel her soft tits envelop your dick. Warmed from both the friction and the spit, you sigh contentedly. Sarah's busy tongue laps at the head every time you thrust forward, and you grin in excitement.

Your thrusts increase with speed, and soon you are flying back and forth. The friction is doing Sarah some good as well, relieving whatever guilt you may have had. She's panting heavily, biting her lip, and whimpering occasionally. Her eyes are half-shut and she's rolling her head back and forth. The spit has mostly dried up, and the friction begins to get to you. Sarah stops for a moment, greedily sucking your cock. You feel yourself just about to get off when she pulls off and lies back down.

"Fuck my tits, baby," she says between heavy breaths. "Cum on my tits." You moan as you put your cock between her tits and begin fucking them again, fast and hard. She looks up at you with wide eyes, and closes them briefly as her body is rocked by an orgasm. The fact that you just got your girlfriend off by fucking her tits propels you over the edge, and your cum splatters her chin, throat and tits in huge loads.

She lies back and cleans your cock with her mouth, sucking you dry for about a minute. When you plop out, you smile as she gets up her tits covered in your jizz.

"Horny, baby?" she asks in disbelief. "I'm gonna take a shower and get cleaned up."