well first up please understand that anal is not for everyone...some women do find it uncomfortable even after lots of trying...and your BF might need to acknowledge that

mind you...some kinda play and stimulation is generally possible but maybe just not full on cock penetration

but yeah, in my experience for those not experienced the 'having a shit' sensation can be strong...not surprising really...dunno if it gets lesser later on or people just learn to ignore it...I had an ex who always found it to be very distracting we eventually just abandoned anal cos she never got any real enjoyment from it

hmmm...not knowing what kinda anal play you've tried so far...suggest some nice toys...maybe some beads and so on used along with some long, gently cunnilingus :-)

have fun though...that's always the important test