Why do you want to avoid it?
From that standpoint, I can understand because personally I can't imagine them together despite all that fan art I see online which is dubbed "Adoribull" I just don't like it.
I think the only reason why I don't like it is because I'm a fan of Bull and the only person I want to see riding him is my Inquisitor! She will always be is kadan.
All that really happens between them is there will be banter between the two of them if you take them both along and none of it will be mentioned unless you have enough approval from both.
The best advice I can give is if you don't want Bull and Dorian to get together, you'll have to romance one of them yourself or just don't take the two of them together when you head out.