Books with way too much sex often feel to me like nothing is happening. OK sex is happening, but it is the same thing over and over (like saying or thinking "you are my everything" repeatedly).
And this also explains why sometimes books with a ton of sex do work for me. Like this one. So much sex. The protagonist uses sex for everything. Not just passion or love but also for exercise, friendliness, networking, embarrassing someone into not telling anyone they saw him, tactical advabtage in a fight, etc. But by the same token, it isn't the same thing over and over. Each time it is serving a purpose in the story and advancing the plot and usually the characterization as well. This book is a thriller, so it isn't like there wasn't other stuff happening, but this explains why I didn't get bored by all the sex.