You've probably heard that the plot of Ulysses hinges around the fact that Leopold Bloom's wife is having an affair, and on this particular day, June 16th, Bloom knows that she's going to sleep with Blazes Boylan. Sexuality, particularly in its relation to love, is an enormous problem that the text confronts. The question is: if you love someone, why is sex so important? The answer: sex is important, but it's not clear why. Over the course of the book, we learn more about the character's sexual desires, hang-ups, and neuroses than we could ever possibly want to know. At the same time that sex is revealed as a problem, part of the message of the book is that sex is something that's natural. It is not evil and it need not be hidden. According to Ulysses, though it complicates our lives, sex is something to be celebrated rather than something of which we should be ashamed.
The fact that Bloom's sexual desires are always mediated in some way – whether through language or fantasy – shows that he has not yet come to terms with whether or not he and Molly are somehow responsible for the death of their son Rudy.
Molly Bloom has successfully separated love and sexual desire in her mind. Her feelings of love and genuine affection are safely reserved for her husband Leopold, and she is simply using Boylan to satisfy her sexual cravings.