I'm not an expert on the mechanics of sex or condoms and cannot say how often semen gets smeared around the shaft as opposed to staying in the condom tip. But surely it happens a lot. But the bottom line (no pun intended) is that condom-protected sex is safe. You definitely don't need to dwell on it.
That said, in response to "I thought I was being safe": You were only half safe. Since you don't say your partner's HIV status, I assume you do not know. The single most important safe sex strategy for gay men, aside from condom use for anal sex, is knowing and sharing HIV status. Condoms sometimes break, and often intentions for non-insertive sex go out the window in the heat of the moment. No gay men should ever have sex with a new partner without asking his HIV status and sharing one's own. Ever. Follow that guideline, as well as condoms for anal sex, and you probably will never get HIV. Do not follow it, and there is a very good chance you will.
Good luck-- HHH, MD
HHH, MD
As for the semen squishing around in the condom, I can't say I've ever really noticed, but I'm sure that's extremely common. Next time, just make sure you leave some space at the tip as a little semen repository ;-)
But in response to your plaintive cry "I just feel I could use a really straight answer. Do I need to worry about HIV in this incident?": NO!
HHH, MD