From what I can gather you were not bothered by the sexual components of your boyfriend's other relationships - but your boyfriend fell in love with this woman without keeping you informed of the changing nature of the relationship, in fact lying about it.
What is the nature of your "relationship" with this other couple (i.e. was it originally intended to be a swinging/sexual relationship only)? Did this relationship come into place before your boyfriend fell for this woman or as a result of it?
Had you and he talked about polyamory and what might happen if someone fell in love with an outside sexual partner prior to this?
On this forum there is a lot of focus on honesty and communication being essential to healthy loving relationships. Lying at the start is certainly a hurdle to overcome - however, if you two had never discussed the possibilities of love entering the equation you may have had different expectations of "when" it was appropriate to disclose that development. Is lying about relationships a pattern of his? Has the lying stopped?
You will find many stories here and advice given to others who were struggling with their feelings about their significant other having feelings for someone else. If you could give us some insight as to what "in particular" is bothering you about his relationship with her people could offer specific advise.
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (22+ yrs)
Dude: hetero poly male, live-in boyfriend (3+ yrs) and MrS's best friend
Lotus: poly bi married female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe