Normally bots automatically copy paste referral link
I don't know where you got that idea, but that's absolutely incorrect. copy & paste is something only humans do. bots operate in computer memory, they don't care.
and even if that was true, do you really think doing something like
ref_address = site_address + "?r=" + btc_address
is hard for a programmer, who can actually beat your captcha?
on the other hand, you're making it hard on your visitors.
just my 2 cents.