The definition of sex according to Google: Sexual activity, including specifically sexual intercourse
Definition of sex according to Merriam Webster: Heterosexual intercourse involving penetration of the vagina by the penis, COITUS, intercourse (such as anal or oral intercourse) that does not involve penetration of the vagina by the penis.
So to answer the title question: lesbians have sex in a way that does not involve penetration of the vagina by the penis. Being as how lesbians don’t have penises penetration cannot possibly involved in lesbian sex (though we will definitely get to toys later.)
And yet, the conviction persists that “sex” involves penetration of a man’s penis into a woman’s vagina. Bill Clinton famously argued that “oral sex isn’t sex,” and he was wrong of course. This is definitely not the case for lesbian couples, where oral sex is one of the most important components of their sexual activity.
Lesbian sex involves lots of foreplay, hugging, caressing, and kissing. Some lesbians use finger applications meant to enhance physical sensations and applications on their tongues that are also ribbed to enhance touch. The sex lesbians have is quite similar to that straight people have with the exception of penetration by a male penis. They will enjoy oral sex and manual stimulation in various positions and engage in sex acts very similar to those of straight couples. Some lesbians enjoy penetration and will add strap-ons to the festivities, sometimes taking turns. Sometimes one partner will be active. Other times, both will be stimulating each other at the same time. Of course, if strap-ons aren’t their thing, there are a wide range of sex toys that a couple can bring into the bedroom. It all just really depends on their imagination and preferences.
It is a common cliché that lesbian couples will have better sex than straight couples because “their bodies have the same rhythm” or “women know how the female body works.” Maybe this is true, maybe it isn’t. It’s very difficult to statistically measure who is having the best sex. Suffice to say that whether you are in a straight or gay couple, effort, communication, and chemistry will always lead to the best sex.