This has been the subject of some debate both on E2 and in rl. This is my analysis of the situation.

The general feeling is that most guys would rather have their partner swallow than spit. The criticism of this is of course: "He got off didn't he? Why should he care what happens to the semen?" Now this is a very good point those males receiving blowjobs at all should be grateful for what they get (so long as their partner is adept enough to keep their teeth out of the way ouch!!!!!). This aside why is it that men prefer swallowing? Here are the reasons that I can think of.

Zero: The stimulation stops. A guy's member is still quite sensitive through and for several minutes after orgasm. A partner who swallows can continue stimulation throughout this time period.

One: This is a bit more psychological. I think that most guys would like to think that their partner is so completely into giving them a blowjob that he/she doesn't even want to come up for air. Kind of an egocentric thing, but hey we all want our egos stroked, especially during sex.

Two: Spitting is generally considered unattractive. For instance: When the kid in front of you turns his head and spits on the sidewalk, your first thought isn't "God he's sexy." is it?

Three: Pop culture says it is better. This is a lousy reason to do anything, but some guys are very conscious of, and very influenced by this kind of thing.

Four: Ease of cleanup. Yep there is little to no mess if your partner swallows it all.

This is by no means a comprehensive list. These are just some realistic motivations that have come up in conversations. I am sure there are plenty of reasons Why guys' partners care if they spit or swallow.