I am afraid I am going to have to answer you question with another question. In a BDSM context "bull" can mean

1] The domina's equal BF

2] The BF of the domina who in turns dominates her

3] The sex partner of the domina in a cuckold scene (often but not always 1 or 2 above)

4] The guy sexually using the sub in a forced bi session

So which one exactly are you referring to? If you answer #1 I am forced to follow up with a question that may sound a bit snarky, "So you have never had a relationship without an element of submission?"

Now your answer is #2 the followup is whether or not you have ever fantasized about switching.

And the follow-up for #4 is whether you have any bisexuality.