Hi, CTC, if you would like to post this as its own question, you will get email notices when someone answers. As it is set up now, Heydelilah will get the notifications. I believe you simply go to the top of the DNA/Paternity community page and click on either "Ask a question" or "Post a question."
Regarding your question, you don't give enough information to say why you would not think your period on May 25 is really a period. What was your due date and when was the baby born?
I had sex with a guy May 3, 2015. Got my last period around May 25th. I had sex with a different guy around the 6th of June...who most likely is the father?
I think it is not going to do any good to stress about this for your entire pregnancy. This is probably what your fiance is trying to tell you. Like someone who obsessively asks with each new outfit if it makes their butt look fat, after a while there is just no point in asking over and over.
It does sound like the guy with whom you had the quickie has much less of a chance, given all the unprotected sex you had been having up to that time with your now-fiance. But since the sex with the quickie guy was also unprotected, you can't be a thousand percent certain that his pre-ejaculate didn't contain sperm.
Ways to not stress about it:
1. Get a prenatal DNA test done with Ravgen or the DDC. Many women don't go this route because they are trying to keep the secret (of having had sex with two guys in the same time period). But this is not your problem, you have come clean with both guys. These tests are costly -- $1,700 to 2,000. But you will know. (If you do the test, do it with both guys. You are looking for a yes, not a no or an I-can't-be-sure-because-I-only-tested-one-guy.)
2. Wait and get a DNA test done after the baby comes, with a reliable lab recommended by the family courts in your area of jurisdiction. (Not a drugstore test, please.)
3. Talk to your fiance about what would happen and how he would feel about staying in the relationship if the baby were to be from the other guy. If he wants to marry you no matter who the baby's father is, trust him and stop stressing. (Continuing to stress if he has said this is an insult to his intentions and his honesty.)
4. Talk to a counselor.
5. Remember that this is going to have an answer; July 4 is coming pretty soon. Nothing good will come of tying yourself into knots at this point, especially if you are not going to do prenatal testing.
Is your worry that your fiance will leave if the baby is not his? How much do you think your pregnancy was what caused the proposal of marriage? If that is what you are stressing about, talk to the counselor about this. The baby's dad is who the baby's dad is, even now. There is nothing you can do at this point to change it, the die is cast. So try to plan for what would happen in either case.