That said, I do think it's interesting that nearly all his strong heroines are motivated by love of a child, and that the tougher they are, the more sexless they are.

There is one semi-exception, in a film he wrote but did not direct, Strange Days. And the difference might very well be due to Kathryn Bigelow as the director. Now Mace in Strange Days is a mother (naturally) and in a flashback it's revealed her friendship with Lenny started when he was kind to her son in a hard situation. And Mace is certainly a classic action heroine - the movie does a total gender reversal in letting Lenny be the one who constantly needs rescuing, and Mace the fighter. Even the one time Lenny rescues Mace happens in traditional "female" fashion - he takes a bullet for her. But Mace's motivation in the movie is not connected to her son, it's her friendship with and not so hidden love for Lenny (who sees her as a friend, and is still in love with his ex wife). Certainly the way Angela Basset is presented in this movie as Mace isn't overtly sexual, but it isn't asexual, either.