Firstly, you don’t need to have sex until you’re ready. I see a lot of clients who fear that they will need to have sex on the first date. This isn’t true.
When you are ready, or if you want to have some action with someone you don’t know, then working out some sentences to convey your needs, and if you need to, a brief explanation of why can help.
Often, when talking about issues, people can go in to confessional mode. They can fear they won’t be accepted and feel the need to spill out every tricky occasion and parents history. Of course this is fine if that’s what you want to do, but it can lead to some people feeling vulnerable. I recommend you work out some sentences before hand that explain what is and isn’t OK. Are you ok with your vulva being touched? What about an inserted finger? As to why, you can say as little or as much as you feel comfortable with. You may want to give a medical explanation of Vaginimus. Or say that penetration hurts or is tricky. Then the person you’re with can have space to ask questions.
If you are up for an impromptu sex session, it’s possible to convey these details without breaking the mood. If the person you are with puts pressure on your or is demanding, then maybe they aren’t the ideal person for you?