With One and Only, I think it's a matter of perspective. If you feel it supports LGBT+ relationships, then great! Especially in JPOP, LGBT fans don't get a lot of recognition in songs so it becomes an honor when a song atleast just hints at something along the lines of the community. If you think it's sung from a male perspective, then great.
There is no right or wrong answer, we all interpret things differently. Unless Tsunku goes out of his way to give us his actual meaning for the song, we'll never know.