You forgot to take into account female-line descendants of the generation 1 men. With your approximations, every generation 1 man who reproduces the first time ends up having descendants going straight down the female line after the first generation.
I derped that one up, didn't I? With the other assumptions, the 90 generation 1 men would always have descendants, since each pairing produces one woman. I guess the only conclusion I can salvage from that scenario is that strictly male-line descendants of generation 1 collapse exponentially while female-line descendants remain constant. I'll work out something better.
There's an idea I've seen a number of times that 80% of women have had descendants, but only 40% of men. A little research tracked it back to this, but the speech doesn't have a cite and I haven't found a source.
The reproduction rates for men and women (possibly for the whole history of the species) seems like the sort of thing which could be found out, but I'd like more solid information.