when men are having issues I think they can turn to porn to try to get it up instead of their partner only because it is more exciting. I don't see looking at porn as cheating but merely needing extra excitement
I was hesitant to respond to capwest, but I partially agree with what Terry mentioned.
First, everyone has their up's and down's sexually. Sometimes it's emotional, sometimes it's physical, other times there doesn't seem to be a reason for it.
I'm a man. A few years ago I went through a period lasting several months or longer where I just had no interest in sex. I was in good shape, happy, healthy, and in love with my wife. I could get erections just fine, but I just wasn't interested in sex. I had my testosterone levels checked at the doctor, they were fine. I started taking herbal medications that were supposed to boost libido, but they did nothing. Then one day my drive returned just as mysteriously as it had gone away. I'll probably never know what caused it, but my wife and I were both much happier.
Advance a few years and now it's my wife's turn. She always had a strong libido, but a year or two after her hysterectomy her drive has dropped to almost nothing. Intense work stress and family obligations haven't helped. We've had some real down times sexually the last year or two, but thankfully we are currently on an uptick again and having lots of fun.
As for the porn, I know some women are really offended by that. Obviously, if it becomes an addiction that comes between a couple it's a problem, the same as alcohol, drugs, or gambling might be. But in general I think men are just wired for visual stimulation. It doesn't mean his partner is lacking in any way, or that he loves her any less. If he still shows you he loves you and isn't sneaking around with someone else, I wouldn't take it personally.
Especially in a long term relationship, it can be difficult to get much variety. I love pizza, but if I had to eat pizza every day I would probably lose interest. Same goes for sex. I like trying new things, having sex in places other than our bedroom, etc. Unfortunately, with a daily routine it can be difficult to find the time or motivation to try new things. It doesn't matter how beautiful my wife is, or how much we are in love, after 30+ years of sex together there's still a certain "been there done that". I don't put as much effort into romancing her as I used to, and she doesn't put as much effort into being stimulating for me. So it can take work to find new ways to excite each other again. Be creative.
Keep in mind there are many ways to have fun sexually without intercourse, especially if there are physical limitations. Try getting out of the routine and do something different, oral, masturbation (on him or for him to watch), etc. Maybe try watching porn with him and "taking advantage" of whatever stimulation he finds from it.
If you are adventurous, make your own porn for him, or be his own personal porn star after learning what he finds stimulating. Note: your idea of sexy may be different than what he thinks is sexy. Men don't talk much, so watching porn with him may give you a bit of insight. Also, preferences change. Something that turned him on last month may do nothing for him now.
There's no easy answer, but it does take effort. It's easy to just let things slide, but if sex is something you want in your relationship you may need to be the one to instigate things if he doesn't. Be patient, it probably won't change overnight, or even in a month or two. If all else fails, at least you can say you tried and can decide where to go from there.
Best wishes!