but it has never stopped me before!

In chapter 4 Henry says, “I am of a cautious temper, and unwilling to risk my happiness in a hurry. Nobody can think more highly of the matrimonial state than myself I consider the blessing of a wife as most justly described in those discreet lines of the poet—’Heaven’s last best gift.” He is trying to be funny of course, but he is just not ready to settle down yet. “I pay very little regard,” said Mrs. Grant, “to what any young person says on the subject of marriage. If they profess a disinclination for it, I only set it down that they have not yet seen the right person.”

Henry is used to women throwing themselves at him, trying to 'take him in' as Mary says. But since he isn’t ready to be married, or hasn’t seen the right person, he is not interested in marriage. He is used to London society, where everyone played the game as described by Mary (flirting and taking each other in just to get married well). He may think the sisters will also 'play the game', since he just wanted them to like him, not die of love. Henry is described as having sense enough to know better than to flirt with both of them, but his vanity takes over. He must know he is not handsome so it boosts his ego to know that he can make a woman like him with just his charm and wit. Also, life at the Admiral's home was not a good example for a young man regarding relationships between men and women. Not that this is a defense, just thoughts on why he would behave this way. He is still very young, maybe he will improve. :-)