Cannot ejaculate with girlfriend, but can during masturbation
Although I usually have no problem ejaculating when I masturbate on my own, I am unable to ejaculate during sex with my girlfriend or if she masturbates me or gives me a blow job. During our six-month relationship, I haven't come once. I have no problem achieving an erection at the slightest provocation and maintaining it for a long time. I'm not on any medication such as anti-depressants and I'm not under any stress as far as I'm aware.
We can joke and tease each other about it, and she says that it's not a problem for her, but several times recently she's said things like "why can't I make you come?", with the implication that she thinks that it's in some way her fault for not turning me on.
During sex, it is tempting to keep going in the forlorn hope that I will eventually ejaculate, but this is not always possible because she dries up and becomes sore once she has reached orgasm.
Could it be lack of stimulation and sensation? Because she is unable to take oral contraceptives, we have never been able to have unprotected sex without a condom. Although I'm not circumcised, my foreskin is usually retracted even when my penis is not erect - could this have led over the years to desensitisation of the head of the penis, such that it's harder to be stimulated to ejaculate?
I'm in my forties but this is my first relationship apart from a couple of one-night stands in my teens and twenties when ejaculation was not a problem - neither premature nor retarded.