This is not good and his behaviour is not acceptable. You are not overthinking it. Your gut is telling you this is wrong.
I am going to say some direct things here.
One of the most concerning things here is lack of consent. Pressuring someone into sexual activity is not consent, someone changing their mind and the other person carrying on is not consent. Someone experiencing pain and discomfort and wanting to stop is the withdrawal of consent.
Another is disrespect and disregard for your consent, opinion, agreement, want….. you said no, so he asks again because what you said wasn’t good enough for him, you say no. He is probably getting annoyed, he ideally doesn’t want to take you by force but he wants to use your body to satisfy his own desires, so he asks again and again. How many times do you think he would ask if you kept saying no? How many nos would it take for it to be violent rape?
Or if this carries on with the pressurisation or if you decide to just let him have you how he wants what kind of horrible disgusting relationship would that be? You would find yourself in a deep dark hole and struggle to be free. When you think about your future – do you picture this, does this make you happy, does this make you smile and giddy with the thought of an exciting future? – I bet not.
Couples have anal sex, couples do bondage, couples do domination and submission, couples do sadism. However, I am pretty sure the healthy relationships, fulfilling and loving relationships are when two people CONSENT, not pressured and BOUNDARIES are RESPECTED.
I question;
Do you walk on eggshells to avoid disappointing your partner?
Does he ever say your perception on present or past events is wrong?
Does he like to know where you are and with who?
Does he ever say mean things in a jokey way? Does he ever put you down?
How frequently do you apologise to him?
Is he sometimes loving then sometimes distant?
Do you genuinely desire him sexually – for your own desires not to please him or because you should?
I imagine your comfortable, your settled, you know what to expect from him - for now, you may not want to be single, this may be easy, you may feel that you can’t find anyone else because of the things he’s made you believe.
Think about what you want your life and future to look like. Is it this? Is it accepting poor behaviour? Is it ignoring the bad behaviour? Is it putting up with his behaviour to have someone? Is it having weekly or more anal sex sessions where you stop crying because your numb? Or is it laughing, making plans with your partner, going to the park or playing with your children together? Is it having exciting passionate romantic sex where you’re just as satisfied with him and desire him looking forward to each time you MAKE LOVE?
Be kind to yourself. You get one life to live on this world. Many have theirs taken away from them. You have the chance to be happy and do the things you want to do within whatever means you can.
You do not want to be an old lady at the end of your days reflecting on what he makes you put up with.
Get out.