Demetrius, it seems to me, doesn't actually threaten physical abuse in a serious way - but more because he's tried more or less every other tactic he can think of to get Helena to go away and stop irritating him:

You do impeach your modesty too much
that
It is not night when I do see your face
Therefore I think I am not in the night...

Demetrius, Helena argues, is too virtuous to rape her - and anyway, because she loves him so much, it doesn't even seem like night, so she isn't scared. And therefore, she isn't leaving him alone.

If anyone *is* serious about some sort of sexual or physical abuse in the play, it's Helena herself:

And even for that do I love you the more.
I am your spaniel; and, Demetrius,
The more you beat me, I will fawn on you.
Use me but as your spaniel, spurn me, strike me,
Neglect me, lose me; only give me leave,
Unworthy as I am, to follow you.

Helena actually wants to be abused by Demetrius - which gives the whole conversation a sort of pseudo-S&M quality. This play really isn't one for little kids!

How does Helena react to Demetrius's verbal abuse in A Midsummer Night's Dream?

You can't help feeling sorry for Helena and the way she is treated by Demetrius whilst he is not in love with her and after Hermia, but at the same time she doesn't really help herself that much. I assume you are refering to Act II scene 1 of this play, when we first see Demetrius pursuing Hermia and Lysander into the forest, and himself being pursued by Helena. We see Demetrius is quite exasperated by the fact that whatever he says or does, even threatening to rape Helena, only seems to make her love for him more constant. Note how Helena responds to the words of Demetrius when he tells her that he "cannot love" her:

And even for that do I love you the more.
I am your spaniel; and, Demetrius,
The more you beat me, I will fawn on you.
Use me but as your spaniel, spurn me, strike me.
Neglect me, lose me; only give me leave,
Unworthy as I am, to follow you.

So, whatever abuse Demetrius throws at Helena, it is only met by fawning love and acceptance. There is something rather pathetic and oddly touching about Helena, who, in her words expressing her desire to follow Demetrius for love, reverses normal classical stories of chases, so that Apollo becomes the one who is chased by Daphne rather than the other way round. She displays the madness of love and is a perfect example of how love causes us to do ridiculous things in its name.