Translation:I love my boyfriend.
Lol. No one says 愛している
Even their idea of the meaning of love in English is just 好き
Ive lived in japan for 3 years, and have met many japanese people. Ive had to tell a few that "love" in English is a very deep and heavy word, and that that word isn't thrown around easily.
Ive also had talks with a Japanese friend, and she believes japanese people don't know what "love" is. (Not my words. But there is a lot of things in their culture that makes what she says make sense.)
In modern Japan, the concept of marrying for love (恋愛結婚 -- れんあいけっこん) has been around for decades at the very least, and even if one finds (/found) a marital partner through a modernized o-miai arranged meeting process, it is not without significant input from both candidates about their take on each other. However, people in various cultures feel uncomfortable expressing love in a particularly direct verbal fashion, or may feel that the constantly repeated avowals of love common in the West seem to be expressions of insecurity regarding a relationship. I'm made to think of the duet in the musical Fiddler on the Roof between Tevye and his wife of 25 years: Do you love me? Do I what? ....
In English, using the present continuous can sometimes convey a shade of emphasis, so I guess this sentence could be legitimate to highlight one’s affection.
I know for sure that it can at least be used derogatorily to criticise an activity seen as too frequent by the speaker, as in “He is always cuddling his kangaroo.” (which was the very example I was given back in the day, by the way! XD).
The word 'love' belongs to a group of verbs that indicate a mental state rather than a process. They cannot ordinarily be used in a progressive (/continuous) tense. Therefore, we do not say: I am knowing this (/you). McD is trying to grab your attention precisely by breaking the rules of English. Languages change over time, so maybe we will be loving or hating that change.
I recently watched a YouTube video of a researcher of English language talking about how English is continuing to change, especially with the number of second language learners and former British colonies around the world. One thing he mentioned was in Indian English it is quite common to use the present continuous tense to say something like "I am remembering it" or "I am knowing it" and he even made the observation that it is creeping into other dialects of English too. He didn't seem to think McDonald's was breaking any grammar rule to grab your attention, and at least for me I hadn't really thought that "I'm loving it" was unusual until I watched that video (native Australian English speaker having seen/heard that slogan for many years since I was in high school).
What strikes me as odd about McDonald's "I'm loving it" isn't that it's grammatically odd, but it seems to have a different meaning than what they're intending. Like, "I'm loving it" implies a temporary state, and I doubt they're wanting to imply "I'm loving it right now, but I might not later on"
You should probably not say あいしています to a Japanese boyfriend or girlfriend. It's probably safer to say 大好き（だいすき） which Japanese people use more often themselves to say "I love you". As mentioned above, あいしています is a very strong, intense emotion and may scare off some Japanese people if the relationship is not yet at that level.