Most hormonal birth control methods work by releasing hormones that stop ovulation. So if you're using the Pill, the ring, the patch, the shot, or the implant — you're not actually ovulating each month (yay, no babies). But your ovaries are also responsible for producing testosterone, which is thought to be involved in your sex drive, says Minkin.
Plus, the estrogen in birth control can increase sex hormone-binding globulin (SHBG), which binds testosterone so that there's less of it circulating in the body, says Gunter. A 2006 study in the Journal of Sexual Medicine found that birth control pill users had four times the amount of SHBG than women who had never used the Pill. So that might play a role, too.
Finally, shutting down ovulation means you won't get those mid-cycle spikes in testosterone that some women blame for being really horny when they're ovulating. Instead, birth control makes your hormone levels stay relatively stable all month long. Not everyone feels those highs and lows throughout their cycle to begin with, but if you used to be sex-crazed around ovulation before you started birth control, you might miss that once you're on it.