You’re changing the premise of the question. We are not assuming the husband “has difficulty getting aroused”. To the contrary, we are assuming there’s no problem there. Only that during the marital act itself, he cannot “finish”. Just as many wives are aroused and “in the mood” for the marital act, yet cannot reach climax during the act itself…no matter how much they wish to.
So saying the husband has to “finish inside the vagina” is useless - he would if he could, but is unable. So what then? Only options are: 1) Abstain from sex altogether. 2) Continue to engage in the marital act, and reach his climax afterwards with his wife assisting, through the use of other acts (basically she would be masturbating him). 3) Continue to engage in the marital act, but resign himself to never achieving a climax.
So after the marital act is over and complete, in order to reach climax the wife is morally free to engage in what is clearly masturbation, with her husband’s assistance and (probably, nowadays) that of some battery operated toy? All the early saints and Church Fathers would have called her a prostitute for this. Of course, that doesn’t mean they’re right (not an infallible pronouncement). But still, it gives one pause.
You can see how very, very complicated this all gets.