For me 100% - STD! That I don't want to get)
For me it's STDs. Pregnancy is hard & scary, but I watched my Uncle die of AIDs when I was 12. If everything goes okay with a pregnancy, then you have chance to share a life. If something goes wrong & you get something like HIV... Then again childbearing can be just as lethal.
Either way, protected sex is the best preventative for both!
I think both would be of great concern, so to avoid this feeling of fear, concern and long term consequences of unprotected sex, always practice safe sex and I recommend regular medical check ups also.
I´m a male, so my feelings may be influenced also by that fact, but as for me, when I had unprotected sex with a woman, the idea of pregnancy was more scary to me? Reasons:
-the probability that the woman was fertile was much higher than the probability that she had STD (statistically speaking, if you count all women in population who could get pregnant and all women in population who are infected by STD, I guess that the first number would be higher, at least in my country.)
-some STDs can be cured, but pregnancy cannot be "cured" (abortion seems to be quite drastic and brutal "cure" and unwanted child is the worst case, even worse than my own death in case of catching HIV...)
-if I got infected by her STD, it would be my "bad luck", but I would be the only victim who suffered as a result of my irresponsibility (I suppose that I was healthy and she had STD), while if she got pregnant, my irresponsibility would lead to three victims: the foetus/child, the woman, me.
Note - my answers are just personal reflections of FEELINGS, probably irrational feelings, and in no case should they be viewed as based on "scientific" or "rational" thinking...