You might also consider showing her the link to our readiness checklist, so you have more tangible things to discuss than "it's just a bad idea." You could ask her to look at sections on it where she feels like you don't have what you probably need yet, whether that's about access to sexual healthcare, communication skills, or even about where she might feel your boyfriend doesn't have things yet.
You can also ask her to be more specific about why she thinks it would be a good idea when you're 16, but a bad one now: what does she think or know will be different for you then? What does she feel like makes whatever kind of sex you're already having seem like a good fit for you now, but wouldn't be the same with intercourse?
With these kinds of conversations, it's really helpful for everyone involved to be more specific than vague, and to talk about things calmly and with clear communication.
I do want to add, in case it's something you want to know or consider for yourself, that 14 is certainly on the young end when it comes to intercourse: on average, that's a kind of sex more people now are only starting to be part of (when they are at all) around 17, rather than 14, and part of the why with that often is things like that managing contraception, STI testing -- for both of you -- managing your own transportation and costs for those things, sexual communication and the extra risks intercourse involves other kinds of sex don't, or don't present as high of risks of is really challenging for very young teens to do, and does tend to be easier to manage with just a couple more years. And of course, when it happens, an unplanned pregnancy is a lot more challenging and hard, per your health, and practically and financially, for someone 14 than for someone even just a couple to a few years older.
One more thing you might want to look at -- alone or with your Mom -- is this:
Sorting Maybe from Can't-Be: Reality Checking Partnered Sex Wants & Ideals. Something we know from study is that the younger a person is, the more unrealistic their expectations tend to be with sex, especially intercourse (like thinking that it's going to feel, physically or emotionally, bigger or way different than whatever kind of sex you're already having, when really, the only big differences tend to be increased health and emotional risks).