My boyfriend has a weak erection. Penetration is impossible without oral sex. What is the reason for this? What to do? Maybe I don't excite him?

The consequences of anal penetration, most likely. Ask, what if your sexual partner had someone of the other sex and already reacts badly to a beautiful female body (I don’t know what it is like there by his standards), or maybe it was a woman? In any case, three fingers inward should smooth out your difficult situation.

There can be many reasons and do not rush to look for a problem where it may not be. In such matters, for a start, it is important to collect anamnesis, clarify the age, duration of the relationship, whether a similar problem has occurred before, etc. This is not very impersonal information and it may be uncomfortable for you to talk about it in public. To talk about the different options that this may not be very appropriate and also not very appropriate. If you have a trusting relationship with your boyfriend and you are confident in your feelings for each other, the best option would be to contact a sexologist and find out what the problem is.