I couldn’t find an found the Archive.org archive of the page from Nullsoft’s website, but this page on some-random-website-I-am-not-sure-about-in-any-way and it has the page saved for sex.exe and its source code which is indeed FOSS:
https://www.1014.org/code/nullsoft/sex/

https://web.archive.org/web/*/http://www.nullsoft.com/free/sex/

Copyright (C) 1998-2000 Nullsoft, Inc.

This software is provided 'as-is', without any express or implied warranty. In no event will the authors be held liable for any damages arising from the use of this software.

Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it and redistribute it freely.

Edit: And the updated version, SafeSex:
https://web.archive.org/web/*/http://www.nullsoft.com/free/safesex/
but I don’t see the source code on this page.