Many of my therapy clients have been people struggling with non-consensual sexual experiences. Even people who come to me with different types of issues (anxiety, depression, grief) frequently describe difficult sexual experiences in which consent was absent or unclear. One of the most common things I hear about is women who have ended up having sexual experiences with someone in whom they had a romantic or sexual interest, but with whom they were not ready to have sex. Often they are unclear about whether they actually consented. They may not have fought back, or yelled "no". As a result, they often they feel that they must have consented. And yet, they did not enjoy the sex, or want it to happen, and they feel scared and violated afterwards. We often talk about whether that experience was rape.

While I haven't spoken to many of the people on the other side of this experience, my guess is that they choose to interpret the silence of their partners as consent. This shows a lack of concern about whether their partner is enjoying the experience, and leads not only to some pretty bad sex, but also to serious crimes like sexual assault.

The advice I would give about handling this type of situation is:

Be clear with yourself about what you want. For example, if you're heading off to a party, ask yourself whether you're hoping to hook up with someone that night. Or maybe you really don't want to have sex. Or maybe you're hoping to meet a special person you could have a relationship with. You can always change your mind later, but it's really helpful to be clear with yourself about your expectations or hopes before going into a situation.
Communicate early and often with potential partners. For example, if you meet someone at a party and you hope to have sex with them, let them know your intentions clearly. It's good to ask at each step. For example you could say, "Would you like to go somewhere private?" and then later "Would you like me to undress you?" This is not a turn off. It's sexy to be interested in what your partner wants! Think of it as a challenge to ask for consent in the sexiest way.
Be prepared to take no for an answer. I know it can be disappointing. But if you go ahead with something sexual that your partner doesn't want to do, then you are harming them. Potentially in a really big, long lasting way. Is that what you want?
Remember that if a person wants to be persuaded, they will let you know. Sometimes backing off when someone wants you to can become the biggest turn on.
Don't assume that because you've gotten consent once, for one activity, that any other sexual activity is also OK. Keep asking. You don't have to be obsessive or weird about it and take all the attention off of the good feelings. But a few check ins ("Do you like this too?" or "How about this?" or "How are you doing?") can be done in a sexy way, and will likely be super attractive to a partner.
On the other hand, if you meet someone and you don't want to have sex with them, it's good to let them know that pretty early too. For example, you could say something like, "This is really fun, and I like you a lot. I also want to be clear that I don't want to have sex tonight, just so you know."
It's OK to request that your partner check everything with you before they go ahead. You might say something like, "Hey, you know what I love? I love it when you ask me about what you're going to do. Like this: Is it OK if I do this?" It can be playful and very sexy.
If someone starts a sexual activity with you that you don't want to do, be clear about what you want. You can say, "Hey, I'd rather not do that?" If you're worried about offending or turning off the other person, maybe you can offer an alternative that is OK with you. "What do you think about doing this?"
Be aware that some people will not pay attention to what their partner wants in terms of sex. Some people will push your boundaries, and some people will cross right over them. It's good to think ahead of time about being in that type of situation, and what you might try do. And remember that in that scary moment, your nervous system might make you freeze. It's not your fault when this happens. It's your body trying to keep you alive.
It isn't always clear who will be respectful of your wishes during sex and who will not. I don't want to freak you out, but people have been raped by close lifelong friends. It can be a good idea to be cautious about being alone with someone.