What does it mean when your boyfriend wants to find a "strictly platonic" relationship with another female?

I went through practically the same thing. My boyfriend of 4 years went on a website and met a girl and was talking to her and I found out through lies too. He said he just needed a friend or something but I ended up finding out there was a lot more flirting than was being admitted. I know not every guy is the same but something like that does seem sketchy and I wouldn't trust it. Let him know it makes you feel uncomfortable and there are plenty of people he can be friends with that you both can get to know. If he cares about you then he should have no problem letting go of some random online girl. Hope it helps a little.