I want us all to reject the premise that such penetration is a qualifying factor in what constitutes sleeping with someone else. It's a Neanderthal-ish ideal that sex is something one person "does" to the other; a facile transaction with a beginning and an end.
What does non-penetrative sex mean? It can be kissing, any kind of oral sex, or what is normally known as "mutual masturbation". It can range from dry humping to erotic massage to use of toys, and much more. For the modern generation, I'd also like to throw explicit sexting into the mix here, too.
From those I've interviewed about this topic, people from religious upbringings seem to often define penetrative vaginal or anal intercourse as "sex" while other kinds of sexual contact aren't taken quite so seriously to them. My general understanding from this is when exploring their sexuality, teenagers from religious backgrounds may draw lines concerning the sexual acts that are permissible pre-marriage, and which should be "saved" for matrimony.
Faith underlines a lot of society – even if you're from a non-religious background – so I wonder if this distinction is why some define intercourse as the only "real sex" while other types of sex are peripheral (and perhaps less serious). I also ponder, if non-penetrative sex is "less serious" in the eyes of some, does that make it "less shameful" to them, too? These are interesting questions I don't have answers to (but I also think no god has any place in the bedroom, so I'm not an informed authority on it).
Secondary to this is the (perceived) larger emotional aspect of intercourse, as opposed to non-penetrative sex. Emotional bonding can absolutely be a part of penetration, yet I fail to see why other sexual acts are seen as less meaningful when it comes to human connection. Physical contact is physical contact, skin is skin, and touch is touch. I personally don't think one needs to be "inside" another in order to display/strengthen that emotional bond.
I even think there can be something less personal about intercourse than other kinds of sex, depending on the individual you're with. I talked about that "transactional" activity earlier: some people approach sex like animals, where the objectives are to get another human being horizontal, achieve orgasm and then roll over. You'll forgive me if I generalise, but these sorts of individuals are usually men.
I'm hesitant to discuss how penetration versus non-penetrative sex fares when it comes to infidelity, because this is unique to a person's views on sex, monogamy, open communication, intent and deceit.
Instead, I'll just leave you with this: When a couple is in a relationship, there is a lot to be commended about mutually-beneficial sex that doesn't involve penile insertion. Not everyone has a penis, wants a penis, gets ultimate pleasure from penetration, or indeed enjoys it at all. Non-penetrative sex is not less important or less meaningful sex, unless you as a unique couple decide it so.