OP, there's some great wisdom in this thread, but given that you thought about it enough to ask the question, you probably already knew the answer. You just needed some help admitting it. In good relationships, you support your partner in their interests, even if its not your thing. So even if she truly is afraid or too physically uncomfortable to ride 2-up, she can still come watch you at a track day, buy you gear for your birthday, wish you a fun day when you go for a ride & meet you at a bar after, etc. And you should do the same for her. If its a problem now after 3 months, its only going to get worse. Time to cut & run!