‘it’ is having sex
when was the last time you had sex

"it" means sex

the girl replied that she cant remember the last time they had sex, she also said they want to do it "last Saturday" but unfortunately her mother came so they didn't do it.

Girl: For him, it was just lust, that son of a bitch
Sender: Should I ask? <laughs>
Girl: When was the last time you did it?
Sender: That's something I wanted to make her guilty about. I could not remember when the last time was, it has already been a long time anyway. We were supposed to do "it" last Saturday but my mom arrived [in our place].
Girl: So funny! You could have just rescheduled it!
Sender: Damn you, you're making me remember the pain again.

"it" seems to mean sex. The girl was asking the sender if the sender did "it", the sender said no.

"They're* just all about lust
Fuck that hahahaaha"

"Should I ask? Hahaha"

"When was the last time you did it?"

"I actually wanna make them* feel guilty
I don't remember, babe. Hahaha it was so long ago. And we were supposed to do it last Saturday but mother dear arrived so"

"HAHAHAHAHAHA FUNNY 😂😂😂
Just reschedule? 😂😂😂"

"Fuck you I'm hurt"

Notes:
*using they as a pronoun because I'm unsure of the gender of the person they are talking abou