If you are debating this question with your partner right now, you're probably looking for a clear yes or no answer. Spoiler alert: it's more complicated than that.
If I were having a one to one conversation with you, I'd ask for your opinion: "do you think watching porn is cheating?" Really, take a second to think about it.
The reality is, what is considered cheating is defined by you and your partner(s). Many of us learn that there are certain things that are considered cheating and certain things that are aren't. The truth is though, there is no universal rule and it is up to you and your partner(s) to decide what is considered cheating in your relationship.
For some people, sexting is considered cheating and for some people it's not. For some, flirting is considered cheating, and for other couples it's a go. In some relationships, it's cool to date and have sex with other people and in some relationships that is considered cheating. You get the point— there is no rule book for what is cheating and what is not, you have to decide.
If you're looking for resource to decide with your partner(s) what is cheating and what is not, @afrosexology this is a great one.
Now, when it comes to porn specifically, it's worth exploring where our ideas about cheating and porn come from. Here in the US, we live in a culture that tells us we are supposed to meet all of our partners’ needs— especially their sexual needs, and if we can’t fill those needs, well, then we are incredibly inadequate (or so they say).
This narrative is particularly pervasive around masturbation and porn. Many people feel as though their partner masturbating or watching porn means that they are not good enough for them. And this feeling can be very strong and unpleasant. The reality is though, sex with yourself is different than sex with someone else. It's not better, or right, it's just different. And that is because it fills a different need.
For example, I masturbate for all sorts of reasons— to try to get rid of a headache or menstrual cramps, because I want to have a quick orgasm, to have some alone time, or to connect with my body in a sensual way. Masturbating, for me, is very different than partnered sex. Sometimes I will desire partnered sex, but not want to masturbate. Or sometimes I will want to masturbate, but not want partnered sex. It's because they are two different things that meet two different needs. And it has nothing to do with my partner's ability to satisfy me— it's just about what I need in that moment.
On the flip side, there are actually many ways that porn can be used to enhance sex and intimacy for couples. Porn and masturbation can be a fantastic way to cope with desire discrepancies between partners. It can also be a great way to explore sexual interests. You can watch porn with your partner to learn more about each of your likes and dislikes. You can also use porn to explore your sexuality. If you are in a monogamous relationship and questioning if you're bi, for example, watching porn can be a great way to explore this.
Ultimately, what you consider cheating is up to you and your partner(s). But, when we’re talking about porn, ask yourself what beliefs are informing your decision and where these beliefs come from. When used in certain ways, porn can actually be a very giving tool for desire discrepancies, learning about likes and dislikes, or exploring fantasies and identity.