It's true that this could be considerate as rape, but even a relationship between guys is considerate as bad in some parts of the world. Stories are fantasy and freedom to those people who want to express something. Couples with a huge age gaps are not really common or seen as good, but the reader can understand their problems more and it becomes more interesting than it would be with generic couples. Most of the people seek something they can't have.
In some countries it is legal for a 16 year old to sleep (or marry) an older person, even if they would be and adult at 18. I think as long as it's true love and a manga than you shouldn't be too much thinking about the plot itself.
I don't undertsnad why you think that the first couple didn't love each other. Of course it ended up having the Uke taken adventage of the Seme, but he seems to care about him. As his Monologue started and he said something about why he met him in the first place (like he just wanted to see what kind of guy he was. He didn't intented to greet him personally). I just think that he feels "something" for him, it's not just the sex.
Anyway, for a starter in the Yaoi community you really have picked something "special". There are some other Mangas out there with a light-hearted story or more romance and good sex scenes. "Smut" would be a good start actually, if you really want to see lots of sex.
I'm new to yaoi but I think some of the critiques here are a little besides the point. I read a lot of romance novels before yaoi. Within that is a house of diff styles that carry different expectations and we judge accordingly. Broadly speaking, with "romance" proper we expect the main couple to get True Love and Happy Ending. Erotic romance ought to come with a heightened heat factor but still end in True Love etc. Erotica is not considered to be romance -- the writer can choose to make it romantic but the MCs may have sex with (a lot) of other ppl, it may end tragically etc. And the darker it goes, the more extreme the scenarios.
To complain that the characters don't truly love each other in the 1st story seems odd to me when it's clear the mangaka wasn't interested in writing a romance. This manga doesn't even carry the romance tag. It's text book erotica really--enough plot to connect the sex scenes but not enough to get in the way of the main goal lol.
It just seems to be a little odd to me where we draw the lines. This *is* a story about an adult man who consciously pursued a relationship with an underage high schooler...which is...you know...statutory rape (yes, even in (some parts of) Japan).