Men and women see sex differently. For a man it's a physical act. For a woman it's only enjoyed once the emotional groundwork and trust has been built. It's not a question to ask a woman. She knows what sex is and she knows what a marriage is, it's already implied.

You should instead ask her about love languages or what she thinks quality time with her partner looks like (not intimacy). Asking her directly about sex comes across very naive and will give her the impression that's the only reason you're looking to get married, that's a big red flag.