Two wrong doesn't make a right - do not accept him within if he
is willing to destroy this three years relation with her girlfriend
because you will never able to recover from the critical blow that
would give you. 
Reputation crash, under social vicious gossip, under strange stares
- this is psychologically damaging and even if that guy is with you
- he won't do a sh|t to save you from this. TRUST ME - he will only
carry the name of being a two timer - while *you* will carry the
name of s|ut, b|tches, wh0re, etc. (Pardon my words, because of the
seriousness of this case & for your sake, I cannot be soft on
my words.)
Paper cannot prevent fire from burning - it is sooner or later that
they might find out. I strongly suggest to take a counter route -
which means you have to speak with that guy and throw everything
out - work for a solution as in you don't want to see him kind of
shit - be firm or you will lose. You see - the thing is that you
*cannot* breathe a word to anyone, except here - where your
identity is a secret. If is very killing to your emotions - but
think again, you could speak to him, who is partially responsible
for this fated incident to happen. At the very least when you blast
everything to him - you will get him to avoid you *OR* at least you
will feel better when you and your best friend are together with
their boyfriend. 
Woman are sensitive as hell - if you don't do that, your secret
will be exposed sooner or later. You could blame it on PMS for your
behavior now, but don't drag it too long. By blasting to him - you
shift partially the guilt to him - because you are now carrying the
burden. (I deduce from his conducting normal behavior like visiting
to your workplace, without feeling very much awkward or different -
which again shows how irresponsible and lame he is.)
Your feelings is there because you have a crush and he *knows* your
weakness. This meaningless ONS that happen only brings out the
womanly insecurity, confusion and more love for this guy, which is
all *illusionary*. Woman needs to be emotional satisfied when it
comes to sex, not just physical - so when that doesn't happen,
gulity often take place.
Prays. (You could pte msg me, if you need to ask anything)
