Ari raises an important understanding - one that often only comes with experience and analyzing.
Basically people have a variety of sexual sides to them. These different sides seem to require compatible (with that side) partners. It's entirely normal and acceptable for some of these desires to NOT be shared with any given person/primary/whatever.
Don't take it personally, or as any false sign about your sexual compatibility etc. It's only one small piece of a larger, more complex puzzle. Celebrate the elements you two connect on and let the other take what direction it needs. The one primary concern you both will want to talk about and be in agreement on is SAFETY. It will be important for you to feel she will be safe in exploring the various pieces of her sexuality. Ask (and expect) her to work WITH you to see that she is in a safe environment. More than likely, once you really have that feeling you will have less concern about her exploration.
In the long run the needs she get's met elsewhere will only enhance what is special between the two of you.
Work it through together - it's all good.