I would honestly advise not seeing your crush before you see your girlfriend because it will probably be seen as a breach of trust; no matter how honest you are after the fact, it still remains that you will be asking for forgiveness instead of permission and as she is your primary relationship at this moment, her needs come first.
You two need to sit down and talk things out. Revealing fantasies can be difficult - we feel vulnerable, the slightest flinch or probing question can lead to us feeling rejected, and we don't always know exactly what we want and describing it can be difficult.
I think everyone's knee-jerk answer to a new fantasy is 'no' or 'what the hell made you think of that' because it's unknown and they've been told at least once it's taboo.
Personally speaking, I've found those things become less taboo as we get older and the usual things we all start out doing in the bedroom eventually become stale or routine. These are prime opportunities to bring up something new.
Now, if it's something that you absolutely have to have to get off you probably need to realize this about yourself and be honest with everyone involved. There are successful relationships where people have separate play partners for kink the other partner just can't handle. But, those relationships require strict boundaries and trust is very, very important.
So my advice is, that if you want to get what you want out of this situation you need to start out being as ridiculously honest and compassionate as you can be with yourself and the people you've mentioned and keep everything open and the communication flowing. Otherwise? You're just going to end up in a clusterfuck, proportions of which you've likely never seen.