i know my wife (then gf) at age 23 and she is age 19.
I go to her house often and it is obvious that her mom likes
me...as i probably look honest and stable....
how course in reality i am like any other guy ha ha ha ha 
anyway, gf and i did think about going overseas together.. but we
know it is hard...
so we do it in sequence...
when she is around ur age 23, the two of us, with her sister and
her sister's boyfriend went to Malaysia... on a short trip.. one
night in Penang only... and then come home...
we stayed in seperate room.. boy-boy, girl-girl.... her mom is not
too happy but go making her and her sister promise that they don't
share room with the boys...
anyway... the next step, my gf insisted on going to HK with me
only....
her mom object... actually MY parents are not too keen on the idea
also.. although at that time, we know each other for like 4 years
already...
anyway, she finally relented...
after that, quite easy already... to go.... because there is
precedence...
and to be honest... if a couple want to have pre-marital sxx,
doesn't have to go overseas together... in singapore also have
opportunity....
so in a way, there is "nothing" your mom can "do" to "protect"
you....
however, she might be thinking of safe-guarding your good name..
etc...
i would propose a few things..
a. does ur mom know your bf well? does she trust him?
b. did your mom shared with u thots of pre-marital sxx? did u share
with her b4? for eg. when watching a tv show... you say say
something like... "aiyah, this young couple should wait until
marriage... etc etc...."
c. instead of bangkok with one other couple, why not start "small"
first.. maybe go bintan with a big group in which your bf is in a
group.... you know, nearer location and bigger group.
d. and then progress to two/three couples and two couples and
finally just the two of u...
e. assure her proactively that u and your gf is sharing a room and
the 'boys' will be in another room... this will show that u
understand her concern.... and she will be happy.