Sex is a powerful thing. It can drive some men to all kinds of
unreasonable behaviour and say all sorts of unreasonable things.
Sometimes it's better to let go than give in. Especially if there
are other issues involved, though they may not be major ones at
this point of time.
I feel a good strong relationship needs to be built on trust and
respect. If your boyfriend truly loves you, there are other
alternatives for him to consider to relieve his sexual needs for
the time being. But he's chosen to go the route of threats, blaming
you if he strays and saying it's your fault that the relationship
lacks passion. Worse, with your keeping to your principles which is
an admirable thing, he seems to be losing interest in you. If this
latest ploy of his is just to force you to give in to his demands,
he is adding to the emotional abuse he has been heaping on
you.
If you are looking at this relationship with a view to marriage,
perhaps you might like to consider taking a short break away from
him to sort things out in your mind? It's not uncommon for men of
this nature to stray in their marriages and leave their families
high and dry. They would blame their wives as well.