The question is worded as if it contains a universal truth. In reality the question should be "why do SOME straight guys..." Unless the OP has had sex with ALL straight guys and has made this observation on the complete set it is not scientific.
Nevertheless, it is an interesting question. My last boy friend had no problem kissing after sex or going down on me after sex. He would usually fall asleep and be too tired to go down on me but it was not because of trace amounts of his sperm.
My s/o avoids his own cum like the plague lmao
I alwasy was curious abuut the taste, I kissed many times my first gf after receving blowjob, just to show her I was greatful of this.
When I met a naughty lady, that loved to give me head and swallow my cum, I surprised her one night by asking not to swallow but kiss me and pass it back to me and I loved it!
To answer your other question I love it when a woman plays with my ass. There you are getting an amazing blow job and you think it can't get any better. Yes it can when she starts playing with your ass.
I know a lot of men have an aversion to any kind of ass play but so do a lot of women. I was dating a woman a long time ago and I brought up the topic while having sex. She was completely revolted at the thought. She got up and put her clothes on and left. Yep, broke up with me on the spot. Probably so much the better if she's so super strait.