First of all, all men do not look at p 0 r n, so it is not a good idea to generalize all ppl.
Second, the reason your husband is playing with yourself, spanking the monkey w/you home is because he is looking to get a good cllmax. he may be having sex with you, but he may not be cllmaxing the way he wants. self stimulation gives very great powerful, extremely stimulating cllmax that one does not always get in regular sex.
In regular sex, you have to be in a good position and the friction has to be right, and it is not always right. plus, he may want his cllmax, but not want to put in the xtra work it requires when he's having sex w/you like getting and staying in a good position, spending time pleasing you etc. These are just examples. He needs a gooooood cllmax is why he's doing this.