My girlfriend has the Nexplanon and we were wondering if I could ejaculate inside her without her getting pregnant.
Nexplanon is over 99% effective at preventing pregnancy, even if ejaculation occurs in the vagina. Remember, hormonal birth control methods do not protect against STIs; only condoms and dental dams can protect against STIs.
Keep in mind that the first week after getting a hormonal method like the Nexplanon, a person is not fully protected because their body is still adjusting to the new hormones. So if you choose to have sex during that first week, please use a backup method like a condom (or abstain from sex until 7 days have passed).
Is there a certain number of times my boyfriend can ejaculate inside me before my birth control becomes ineffective?
Sexual activity, regardless of how often it is occurring, is not going to interfere with the effectiveness of birth control.
Birth control is only going to be effective if you’re taking it correctly, and that varies based on which one you're taking. For example, if you're on the pill, you have to take it within the same two-hour window every day. If you're on the Depo shot, you have to remember to get another shot every three months. These are the things that affect whether your birth control is effective; it has nothing to do with the number of times your partner ejaculates inside you.