There are many ways of saying 'lover' in Tagalog.
In popular usage, a kalaguyo means a lover outside marriage. Your wife is your asawa ('spouse') or misis, while the other woman is the kalaguyo. Sounds wicked in Tagalog. Another Tagalog word for 'mistress' is kerida from the Spanish querida, meaning a cherished one. It sounds nice in Spanish, but not in Tagalog!
A kasintahan is someone with whom you have promised love. There is no sexual connotation to this word, which sounds very pure and beautiful and sparkly in Tagalog. It derives from the root word sinta, an old-fashioned term for 'love.' Pasko Na, Sinta Ko is the title of what is arguably the most romantic Christmas song in Tagalog.