The only reason I can think of where she might let him kiss her is, that she wanted to find out if she feels similar to what she felt when she kissed Akko. So far, Mari has no comparison what a kiss feels like and the feelings she had when she kissed Akko might be the same with everyone else for all she knows.
So, I can think of 3 scenarios of what might've happened:
1.
They kissed and Mari came to the conclusion it's just not the same and not right if it isn't Akko and told the boy: "Sorry, I just can't feel it. It's not the same. I have to go." And left him standing there.
2.
He kissed her and she didn't like it, slapped him and run away. (The cliche romance-manga version.)
3.
He tried to kiss her but before he could she turned away said: "Sorry, I just can't do this." and she run away.
I would prefer scenario 1 because she actually has the comparison and with this knowledge deliberately chose Akko over a boy, but 3 would be ok for me.
There's no way Mari slept with the boy. Mari lied to aggravate Akko and get a reaction from her either good or bad, to know where she stands. I refuse to think of any other option.
