...picking one of the four 'Strategy' starting researches will re-enable recruiting them...
Sadly, i can't figure out how to get new marines at all. To the point when i try playing with only guardsmen in marines strategy research threes. Maybe it's a glitch? I tries to go long enough in 'Scout strategy' and i got to 'Tier II Equipment' without any new marines. Now i start again with 'Tactical' but again - no purchasable marines at all. Maybe i can't get the right research three? There is my save file for example.
Also, i got some fun glitch! Not sure, it's about 'IG Operations' or base mod, but... i have a FEMALE SPACEMARINE! Here's a screenshot. And save file also.