Wouldn't you clench or if somebody did it and you weren't wanting it the other person would know because the anus wouldn't be relaxed enough.
I don't think many rape victims are "relaxed enough", yet they get still penetrated. It's called force.
Or perhaps, more in line with the op & her bf, he isn't expecting it. He will be relaxed to a certain extent if he is being pleasured & it won't be too hard to slip the tip of finger in. But essentially IT DOESN'T MATTER HOW RELAXED HE IS, HE DOESN'T WANT OP TO DO IT.
It seems to me like he might be going along with it because he knows you love it so much. YY, and again, role reversed, man would be called all sorts for forcing something on his gf that she wasn't sure of.
Lots of women don't stop their rapists while they are being raped as they feel powerless, many women don't stop their husbands/bf from performing a sex act on them as they know their husbands/bf like it & "want them to be happy" doesn't matter...if the receiver is not getting any pleasure from it, they aren't doing it for the right reasons.
The ONLY way to handle something like this, when met with "I am not sure I like/d that" is to say "sorry, I won't do it again unless you ASK me to" and then IF they ask you to, ask them if they are sure!!