I wasn't exactly sure how to phrase the title but in yaoi animes, why does the uke refer to their love I guess as senpai?
For example, wouldn't it make more sense to say 'I love you Takano' rather than 'I love you senpai'?
Is this a Japanese language thing or is it better to just say senpai rather than their actual name?
I am going to make a couple of assumptions here. Since Uke is normally the "passive" partner in the relationship, it would stand to reason that they are the younger in the couple, while the Seme, being the dominant partner, is the older one.
In a school scenario, senpai would make sense and I'd assume the Uke would start calling the Seme by their name (probably with the -sama honorific at first) when their relationship is a bit more intimate (intimate = anything after their first kiss).
While I don't read Yaoi, one can't assume that every Yaoi story has the Uke and Seme attending the same school. There may be some where one attends a different school or doesn't attended at all, as such senpai could still be used.
However, the Seme, being the senior, is still guiding the Uke like how one's senpai guides, protects, and teaches their kōhai as best they can. With that, senpai becomes a term of endearment from the Uke after having been guided in the relationship by the Seme.
Senpai means senior in Japanese. It's more polite to call your senior Senpai at school, at work, etc.. rather than calling them by their name alone.
The Uke is sometimes older than the Seme which means it's more polite for the Seme to call the Uke 'Senpai' if he's his senior or call him (Name+San) which means Mr. (Name).
I think it's a situational thing, not necessarily a rule. Like with Takano and Onodera, Takano was Saga-senpai because he was senior, so calling him senpai is just how Onodera referred to him when they were in school. With Koisuru Boukun, Morinaga says senpai because of their senpai-kohai relationship as schoolmates even though he is the seme. It differs with every relationship.
In Japanese culture, everyone defers to titles if one applies, unless you have a special relationship with the person in question (ie, a family member, childhood friend, etc.). Social structure/strata rules in Japanese society. Even within a family, younger siblings call older siblings by their titles, the kids call the parents by their titles. Employees ALWAYS defer to their managers by their titles. Even strangers are addressed by titles when you don't know them because it's rude to just say "you", so depending on your age in relation to their age you use titles like "uncle", "auntie", big brother/sister", "young man/lady". You've probably already experienced this from old, poorly sub'd kung fu movies. Yes, it's not just a Japanese thing - it's an Asian "respect your elders", Confucian kind of thing. You might not always observe this in translated Japanese manga/anime/movies/TV shows because translators switch titles to the characters' names because titles are a little too unwieldy/confusing for Westerners to get use to.
How to address those outside/inside/above/below your particular place in society and how to navigate it is a big deal and that gets reflected in manga/anime/movies/TV. Japanese aren't standing around scratching their heads trying to figure out what to call the other person every time they meet someone by any means - it's ingrained and becomes natural. Just like when you call someone you don't know (or even if you do) "sir" or "ma'am", or when southern folk call you Mr.~ or Miss~ even with your first name. There's a standard of politeness ingrained.
It feels more in control when you are older and your uke calls you senpai as a form of respect. Why? Because uke should respect and obey their seme just like they should with their senpai. It's kind of a,"you're older, you're in control, and I respect that I listen to you" thing.
