Porn is just showing the friction. Sex is so much more. It's the tension building up, the expectation, the heightened senses, the smell, the taste, where one tiny touch can cause goosebumps. I don't know if him watching porn causes her to only want sex once a month. But let's try a different angle. You know why women love romantic books? There is a lot of porn in those too, if you know where to look. But because women want to feel attractive and desired, longed for and see the men make an effort. That can't happen within 5 minutes. If you want to watch porn, try to watch porn made for women. It's different than porn for the male viewers. Try not to skip. Watch it with her together. Make an effort for her sake. Imagine going to a wedding. Now clean yourself up the same way JUST for her. Just for datenight without expecting sex. Make her compliments. Look at her as if she is the most important person on the planet. Talk to her and ask her for HER wishes. Maybe even go to sex clubs without engaging with others and spice up your sex life. Buy her lingerie that you both choose together. Bring the spice back that is NOT shown in Porn. I'm not saying it's all on you. I'm saying try to take both of you to where it's sparkling again between both of you. Find out her love language first for example. For some it's acts of service, others gifts, others touch. Communicate honestly with each other. Maybe she would be willing to have sex more often, but is annoyed that you already helped yourself. Maybe she doesn't and even once a month is one time too many.