Which is odd that some people care so much about this. Games Workshop used to make female space marine models but stopped because they didn't sell well.

My head canon is that with all the hormones to boost mucles, organs changed out, bones merged with ceramics, extra heart, special space marine reproductive organ in the chest, black carapace, acid spit, memory eating organ, etc. Doesn't matter if you start with Judy Garland or Arnold Schwarzenegger you get Lou Ferrigno at the end.