Brief Answer:
No,swallowing semen won't aggravate stomach ulcer...
Detailed Answer:
Hi,
I understand your concerns.
If accidental semen swallowing occurs, then, let me tell you that the pH of semen is neutral to slightly basic (7.0-8.0). Thus, swallowing semen won't aggravate the pH of the stomach, thus, no effect on stomach ulcer.
As all foreign bodies, it might induce nausea or vomiting to throw it out.
Hope it answered to your queries!
All the best!
Dr.Klerida