According to the symptoms you reported, your fiancé might be experiencing a sexually transmitted disease. It is hard to determine the exact cause of an STD without first seeing the bumps in a visual examination and requesting further diagnostic testing, if necessary... but I can give you some possible reasons for the problem.
First, the condition that you've described (bumps around the head of the penis with itching and scabbing) can be caused by both bacteria and fungi. Fungal infections are very common in women, but bacterial infections are usually transmitted via intercourse.
To determine the exact cause of these penile skin symptoms, your fiancé can go and see an urologist and request a urethral swab and antibiogram. The fact that the condition flares up after intercourse with you indicates that you probably transfer an infection to him, but the symptoms are somehow not visible in your case.
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