Logic in the real world

Within the confines of mathematics there is no problem with:

Let X = 1

We are demanding that it be so.

There is also no problem with:

Let Bill = Boy.

The problem with your logical assertion as it relates to the real world comes with:

If Bill = Boy.

Bill might not in all actuality be a "Boy". You can build a whole house of logical cards on If but they are just that: prone to be knocked down.

I believe that Bill is indeed a boy. But there are signs out there that he may be much more than just a boy, in fact, there are signs in the Universe that say Bill is both a boy and a girl (like Cher being both flat and bumpy) and that changes the whole world that I built upon the assumption that he was a boy.

Maybe a more accurate logical treatise for the real world starts with:

If Bill can only have one sex in all dimensions
And If Bill is a boy ...

This should come as no surprise to a Christian ..

If Jesus was a man ..
If Jesus was God ..

etc.