Yes, Magritte’s portrait of his wife is definitely more overtly erotic than Michelangelo's David. I think you could make that case for most of the surrealists in general!
Either that, or it's the lusty way she is regarding the conch shell. Hey, people are into stranger things!
The same goes for Michelangelo's David. Because there is a default assumption of heterosexuality (and gay men are not considered a threat by a protected group), David is not going to arouse suspicion, no matter who is viewing it. Female nudes are a different matter, not because nudity in art is judged differently by the subject's gender, but because the viewer is judged differently because of their gender.
For the NSFW issue (i.e. coworker glanced and reacted badly to what you were viewing), this is just a matter of realism, plus a high sensitivity to what might offend other people and women in particular.
But I also don't think it's entirely about the "different standards to which women are held", which I perceive you to mean female-as-subject. In some ways it's more about male-as-viewer:
As sibling comment points out, when a male views a nude female, it is frequently assumed to be lecherous, or at the very least have prurient motivations.
When a female views a nude male (or female), it is assumed to be incidental, art, or accident.
This is also a double-standard, but honestly it cuts both ways.