Toulon responded before the first half was out through man of the match Juan Smith's try, and though Sexton added a further two penalties, the French called upon their big guns from the bench after the break.
A penalty try and an effort from replacement Anthony Etrillard followed and Tom Taylor's penalty stretched the Toulon lead out to 20-16, which it remained.
Leinster are winless in Europe so far this season but head coach Cullen's attention now turns to a heavyweight GUINNESS PRO12 clash at Munster next Sunday.
"It is just the power of Toulon that unfortunately wears you down," Cullen admitted.
"I thought we defended pretty well in the early part of the second half but they just wore us down, picked up our penalties and kicking for the corners.
"It meant we were defending against big bodies and then you have the power coming off the bench - like Juan Fernandez Lobbe and Mamuka Gorgodze - the list is endless.
"There was a lot of endeavour in the first half but the pure quality they have wore us down in the end.
"Now we need to focus on the GUINNESS PRO12 and then see where we are when we start up again in Europe."
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