Warning: Placing this post under a cut for discussion of sexual content, which I know not everyone is comfortable with, but I feel it's an important post people should read.
As a society, we've done well in teaching people that it is okay to say yes to sex. I'm not going to deny that it isn't still stigmatised in some circles, because it is, but as a whole, it is improving. There is no praise for improving it, as it's something that shouldn't happen in the first place, but it is an observation. Girls aren't shamed as often or called a slut. It still happens, but not as frequently.
However, I still feel we have some way to go to teach people that it is okay to say no. As in, it is okay to say no to anything sexual, for any reason. Boys are assumed to always want it, and experience pressure to act accordingly. Girls who deny their SO's sex are thought to not be 'good' partners. This issue can also effect non-binary people, too, but I am writing this from primarily a cis woman's view.
Jokes abound, like "For Christmas, I'm spending a dollar on my gf for every time we've had sex this year. So far, she's getting a cheeseburger." Wives who don't have sex with their husband are 'that' girl who pretended to be into sex until marriage. Younger women who don't may be called frigid or a tease. Men who don't want to are accused of not being 'real' men.
You can say no. You can say no to sex, or a blowjob, or a kiss, or a hug, or some basic physical touching. You can say no to a certain act during sex, but still be okay with something else. You can say no even though you've said yes before. And a yes to one thing isn't a yes to everything. You can say no because you're tired, you've got a headache, it was a long day, you're too full, you're sore, or dammit, you're just not in the mood. You never have to do it, even if it's Valentine's Day, he's had a bad day and needs cheering up, she just bought you a meal, or expensive jewellery, whatever. No-one is owed sex for any reason.
My first time, I wasn't ready. I was 23 and terrified. But not being ready is something that only effects teenagers, right? Because of societal pressure to say yes, I didn't know I could say no. And it's not that I thought my boyfriend wouldn't listen when I said no, because I knew he would. But you're supposed to have sex. It's normal, right? While yes, it is a normal, natural part of life, it's also perfectly normal to not want it, either.
Also, the answer to a no is not "I bet I can make you say yes," the answer is "Okay, anything else you'd like to do instead?"