In humility, can a somewhat large penis be the reason it never is fully rigid during sex?

Any size gets rigid: Any size of penis should be able to get rigid. Reasons for semi-rigid penises include medical problems, psychological factors, medication or drug effects, past penile trauma, excessive recent sexual activity, older age, fatigue, hormonal imbalances, etc... A primary care doctor or a urologist can evaluate and figure out what the issues might be.

No: All normally functioning penises should achieve full erection irrespective of their size. ED occurs when you can’t achieve or maintain an erection that’s firm enough. For most men over 40, this problem occurs occasionally then increasingly. The causes of ED can be physical, psychological, or a combination of the two. If it is interfering with your ability to engage fully, see your doctor.

ED: Please start w/a visit to your doctor. Follow up w/a clinical psychologist if needed. Cause may be physical (not usually so), psychological, or a combo of both. Peace and good health.

See urologist : A large pens per se is not an explanation for lack of full erection. You ought to consult a urologist for evaluation of your GU and general health.