As long as the signature itself is SFW, and after clicking on it there is a SFW landing page to protect accidental clickers, I don't see the issue. Anyone with exceptionally strong objections due to religion, country of residence, etc, can simply turn off signatures.
I would say that this single precaution would get rid of 90% of any objections over this (if there are any). Maybe arrange with the soliciting site to create a landing page for this purpose.