Our world has set up penile-vaginal intercourse (PVI), specifically, as the “default” sexual activity - as the one kind of sex the majority of people will tend to think of when someone says “I had sex.” This was definitely the case in my adolescence. In fact, there was the notion that it only "counted" as sex if your penis penetrated her vagina. I remember a phrase oft used to enforce this notion, "Yeah, but did you fuck her?" PVI certainly isn’t the only kind of sex people have, it’s just one of many ways to be sexual with someone else, but that’s not how it’s presented a lot of the time.
When something is framed like that, as the thing to do, people will tend to learn, covertly and overtly, that that’s what sex is and that’s what they should want. Strong messages that people should want something tend to pack quite a punch, especially for people who are very concerned with normalcy, which is a lot of people.
Emphasizing PVI does a disservice to everyone who has sex. It makes foreplay seem less important, it pressures people to have penetrative sex before they may be ready, it perpetuates a heteronormative and phallocentric notion of sex, and it limits the amount of pleasure that can be experienced.
Could "I had sex" also mean oral sex? Anal sex? Digital sex? Mutual masturbation? Phone sex? Perhaps it is time to start thinking about outercourse in addition to intercourse.