Yeah one of the previous updates removed the limitation of females not being able to use the breeding yard to have sex if they got prego (Thank goodness it did). It was quite frustrating before having regular females not being able to use it at all when they got pregnant and having futas be limited to be "feminine males" while the same happened to them. Now both can keep having sex even if they are currently pregnant which made the "Get a sterile Neph" process sort of pointless or say totally unneeded. Simply get a Neph (Or nephs if you play without the Infinite Lust cheat) pregnant and they can be the ones that helps you to gain levels.
Tho not exactly sure how with the whole new stats system the leveling works, specially with the vagrant system.
New stats system seems to work like this:
E to B: fuck enough times with Nephs with decent stats, D can get you to C, C can get you to B. E can get you to D, ofc.
Your stat max is 1000, it's also Neph max unless they're better than B. Nephs with stats high enough despite being only E can count as higher than their grade (edit: I'm pretty sure, training you up to B).
A: you'll need another B to mate with. Keep doing that.
Your max is 1200.
S: you'll need another A to mate with and their stats must be above 1000.
Your stat max is 1500.
Stat gradings are transferred to offspring in a similar manner to levelled traits. They appear to contribute towards release value, but I'm not sure about ascension value. Increasing the player's stats is easiest at the start, so the player's offspring is worth more. (edit: if you breed two generations down, while still using E class stat Nephs, the offspring might look similar to you but their stats will mostly return to racial level 1 normal)
It seems more likely to up a stat to A or S for the player by mating with a race of Neph who has that stat high naturally than to do so with one of your own descendants of another race who has the exact same stat grade and level.