Contrary to what some people believe, I see Stacey as being among the LAST of the BSC to become sexually active.
Although she goes through a "boy-crazy" phase early in the books, she's actually a bit shy with most boys.
Mostly, she blushes and timidly starts a light conversation, but doesn't flirt heavily the way some "boy-crazy" girls do.
Also, she's a bit more reserved than several other BSC members, namely Kristy, Claudia, Dawn and Abby.
Also, she has an extremely close bond with her mum and I think she would ask her mum for and get good solid advice on avoiding sexual involvement with the wrong guy, especially too soon in the relationship.
Studies have shown that teen girls who have a close relationship with their mums are less likely to engage in unsafe or impulsive sex or to become pregnant as teens.
Also...as somebody pointed out, she does fret some on what her peers think of her.
And given the culture that pervaded in as late as the 1990s that sex-shamed girls, I can see Stacey being fearful of that stigma of being labeled and sex-shamed, so I see her holding off on sexual activity until her mid-twenties at least.
I see Kristy, Claudia, Abby, Anna, Dawn and even Mary Anne and possibly even Jessi and Mallory becoming sexually active before Stacey.
That's my take and something I was thinking about re-reading several BSC and Girl Talk books.