Hmmmmm... Ok... I can see why their connection is upsetting to you... Based on the composite chart, it does seem like he is very close to her. They have good communication going and there might be an attraction on a romantic/sexual level.
However.... The question is - If they were that into each other - Why would he be in a relationship with you?!
If someone showed me their composite chart and told me they were roommates... my first reaction would be: "Huh? And they're not "together"? Why?" - Because it seems a little odd... considering the Venus/Mars connection, the Moon/Venus square, and the famous "love stellium" they have happening with Sun/Mercury/Venus all conjunct...
Also the Venus in the love stellium ^ is the ruler of the DC.
But a -chart- is just a chart at the end of the day.
It's possible for example that two siblings.. or cousins could have such an intense chart - and things between them would likely never be in any way romantic or sexual. They might just be very close as friends!
Or, another example would be - two hetero people of the same-sex... wouldn't normally become romantically "involved", even if their chart pointed to an intense connection...
So, in "theory" - this could apply to a hetero male2female friendship... It could be a close friendship but non-sexual...
Attraction is not purely based on astrology. There are many other factors, not the least of which are genes.
For instance, a woman could have unbelievably awesome synastry (or composite) with this guy :
But ^ if he's not her type physically... the chances of romance would be pretty low.
So it all depends a lot on how much you trust your bf and your relationship. If he is loving and he treats you right - and he has not given you reasons to distrust him... then I would believe him when he says that - his connection with his roommate is purely platonic.
But... if you generally don't feel like he is a very trustworthy person... and he has given you reasons to doubt him in the past - then it might be time to re-assess whether he is really worth it.
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I'm wondering though.... if you are close enough to this girl to know much about her sexuality... Is it possible that she is not at all hetero - and not even romantically interested in men.. for example?
^ Could there be pieces to this puzzle that you're unaware of?