It would make sense to me that you might have some residual feelings about this, but there are several things here that are important to highlight.
It sounds to me like both of you made a conscious decision to experiment with each other, and that there was also a mutually agreed upon limit to what was going to happen. It was a tacit agreement as to how to handle the experiment with a mutual and reciprocal respect.
These are good things. Two people deciding how to experiment with their bodies is healthy because they are also learning to negotiate and trust each other. It sounds to me like this was a new experience for both of you, and this was a way to test out your feelings about sex and for each other.
Please do check yourself out medically, but from your description it sounds like it was not possible for the semen to impregnate you. In fact, it sounds like both of you may have made the decision for mutual masturbation for just that reason, which, if you didn’t have a condom, would also have made perfect sense and a good choice.
As far as the guilt is concerned I would do three things: First I would write all your feelings into a journal and don’t hold back. Keeping a journal of your thoughts and feelings in reaction to a situation is one of the best ways to get a handle on it. By writing, you get a chance to reflect and organize your thoughts.
Second, if you are willing, talk about the experience with this young man. Remember he was the only one there sharing the moment and he too might have some unfinished feelings. If the two of you are going to have something more intimate with each other, talking about your experience together will help.
Finally, you may want to make an appointment with a counselor on campus. The college counselors are very well trained in helping new students negotiate though these things and can help get you through the guilt.
This was an experiment that had limits placed on it and was mutually agreed upon. Try to accept that these were good boundaries brought to the moment.
Wishing you patience and peace,