Passions are spurs to action, and open the mind; but they sink into mere appetites, become a personal and momentary gratification, when the object is gained, and the satisfied mind rests in enjoyment. (2.45)
It's true that sexual passions can push people into action and even make them do great things. But the truth is that once sexual urges are satisfied, the passion totally evaporates and eradicates all feelings of love.
Still, highly as I respect marriage, as the foundation of almost every social virtue, I cannot avoid feeling the most likely compassion for those unfortunate females who are broken off from society, and by one error torn from all those affections and relationships that improve the heart and mind. (4.66)
Wollstonecraft feels sorry for all the women who have ruined their reputations by having sex with some guy who charmed his way into their beds. This social punishment for a woman having premarital sexytimes is way too harsh in Wollstonecraft's mind.
It does not frequently deserve the name of error; for many innocent girls become the dupes of a sincere, affectionate heart, and still more are, as it may emphatically be termed, ruined before they know the difference between virtue and vice. (4.66)
Many girls are kept so ignorant of the world by their parents that they have sex and ruin their reputations before they're even old enough to understand how they're supposed to behave. In Wollstonecraft's mind, it's the parents' fault for keeping their daughters in the dark.
Love, considered as an animal appetite, cannot long feed on itself without expiring. And this extinction in its own flame, may be termed the violent death of love. (4.71)
When Wollstonecraft says love here, she means sexual desire. She compares sex to a fire that can't burn for very long without going out. That's why you can't base a good relationship on sex alone, according to the wise Ms. W.
These are all preparations for adultery; or, should the fear of the world, or of hell, restrain her desire of pleasing other men, when she can no longer please her husband, what substitute can be found by a being who was only formed, by nature and art, to please man? (5.31)
If you only educate women on how to please men, what do you think will happen when habit and routine have made a woman incapable of pleasing her husband? Of course, she's going to look for some other man to please…
But what serious consequences ensue to rob man of that portion of happiness, which the Deity by calling him into existence has (or, on what can his attributes rest?) indubitably promised: would not all the purposes of life have been much better fulfilled if he had only felt what has been termed physical love? (5.148)
Men are so obsessed with physical love (or sex) that it practically becomes a religion for them. Pretty much everything they do is directed toward having sex, and many of them believe that their lives could be better if they'd had more sex in their time. They also tend to see sex as an entitlement that God has promised them.
This regard for reputation, independent of its being one of the natural rewards of virtue, however, took its rise from a cause that I have already deplored as the grand source of female depravity, the impossibility of regaining respectability by a return to virtue. (8.10)
Wollstonecraft thinks that there is one major reason why women focus more on their reputations than on their inner characters. It's because once you've lost your reputation, you can never get it back. In Wollstonecraft's world, people would never forgive a woman for having sex before marriage. In the eyes of the world circa 1792, that makes a woman "ruined."
But, with respect to reputation, the attention is confined to a single virtue—chastity. If the honour of a woman, as it is absurdly called, be safe, she may neglect every social duty. (8.20)
Wollstonecraft is annoyed by how much importance is placed on a woman's chastity (or her virginity). Wollstonecraft hates this focus because it basically tells a woman that she can be a horrible person or a good person and it doesn't much matter; all that matters is that she doesn't have sex.
Men are certainly more under the influence of their appetites than women; and their appetites are more depraved by unbridled indulgence and the fastidious contrivances of satiety. (8.23)
Wollstonecraft is convinced that men tend to indulge their sexual appetites much more often than women. In the long run this destroys men's sense of morality and discipline, but they're still allowed to get away with it because they're men.
I have before observed, that men ought to maintain the women whom they have seduced. (8.30)
Wollstonecraft believes that if a young man seduces a young woman and has sex with her before marriage, the law should force him either to marry or to take care of this young woman financially. Having sex before marriage would ruin a girl's reputation in Wollstonecraft's time, and she maintains that the young man should be held just as responsible as the young woman.