Is it only by the context?

In other languages, there is a specific word for a girlfriend who is lover and a girlfriend who is just a friend. For example, in Portuguese, you use namorada for the girlfriend who is a lover and amiga when the girlfriend is just a female friend.

So, in Portuguese, a man can say to his wife: "don't worry, she is just my amiga". In English, that doesn't seem to work, "don't worry, she is just my girlfriend"?

I'm very curious as to how native speakers distinguish these two uses in day-to-day life, as it can cause some troubles. Same thing with the word boyfriend.

Also, I guess this has more to do with the etymology of the word, but doesn't calling your significant other the same as a friend make it sound less significant? They are more than a friend, they're not just a friend with their gender specified (boyfriend/girlfriend). There should be a specific word for it. Or, I'm just going too far on my nighttime mind-wanderings. Maybe I should just go to sleep. Night night.