Raiden, you are definitely jumping the gun.
Don't go looking for trouble, it will find you quick enough if it's there at all. Don't trouble trouble till trouble troubles you !!!!
You have only been together for 3 months, she's older than you and is sexually high functioning and she must have thought/thinks she is one lucky lady to find a young man who can keep up with her!
She may really not have meant anything by the comment, other than a poor joke. Only you know what the real state of affairs are. As you feel that uncertain I get the impression that the relationship is based mainly on sex? If that is so then there may be the possibility that she will not be able to cope or help you to cope with the next few months. You never know, this may be a catalyst for formalising your relationship. If it isn't then you are young enough to start again when things settle, but don't write this relationship off just yet.
You are young, you have a good sex life so there is nothing definite to say that you won't make a good recovery and regain what MAY initially be lost.
My husband was 73 when he had his brachytherapy and he was already having problems in that area. Yes, he does have to take sildenafil (viagra) if we want intimacy but we can if we want to.
I am thinking of you. I am confident that your common sense will stop you worrying about the future until you know what you are dealing with. I hope so anyway.
All the best
Sandra