The word in Proto-Indo-European *pes means penis. The other word "fæsl" means (according to oldenglishtranslator.uk) "seed [or] offspring". I can kind of see what the relationship could be (changing the p to an f, and sexual reproduction). Is that theory correct?
Probably. Nothing's ever entirely certain when it comes to reconstruction, but comparison with Latin pēni- "penis" < PIt *pesni-, Hittite pesna- "husband", Sanskrit pasa- "penis", and so on are pretty solid evidence for the PIE root, and PIE *p > PGmc *f word-initially is a famous and uncontroversial sound change.