@Dekunator
there are actually quite a few manga without love triangles, it is in the minority though:
otoyomegatari
the ancient magus bride
bonnouji
tomo-chan is a girl
teaser master takagi-san (same author as this manga)
there are even more if you count ones without romance as focus:
holyland (romance isn't that important though)
fullmetal alchemist
there are also manga with harem endings so not really any romantic rivalry: