Probably allergy, yes. But there was also a horrifying story here years ago, involving a skin reaction to microscopic parasites in the guy's sperm. You may want a dermatologist to confirm that it's just an allergic reaction. (Or just see if you have that same reaction to someone else's sperm. It's awkward as hell, but cheaper than a dermatologist visit.)

Cum on! Do you expect me to believe that? I thought I've semen everything, but you think I'm going to buy all that ****? Hah, I'm no spooge!

It could be a reaction to something he eats or drinks, or if he's a smoker or drinks alcohol. Probably best to see a doctor.

You found a good way for men to ask for such a delicate move: « it’s just for your health babe, to see if you’re allergic ». More seriously, sorry for you, but before jumping to such conclusion, go see a doctor to talk about that reaction.

Not a doctor, but it may be more likely that your body is reacting to a component of semen and not the sperm specifically. And if it's just with your boyfriend, it may be something to do with his diet. Talk to a specialist, and use condoms until then. Good luck!

be thankful you aren't into swallowing. seriously. this week I learned some folks have to make a run to the toilet shortly after swallowing because a protein in the seminal fluid can cause massive bowel irritation :(

have someone else do it, maybe he's diseased...