is it cheating?

Every person has their own limits on what they would consider cheating on them.

You really need to ask you SO what constitutes 'crossing the line'.

Some SO's might not care if you sext as long as your not sending/receiving pics, and don't form an emotional attachment.

Other SO's might think it's hot your sexting others, they might even be interested in making a date +1.

But the general consensus, and majority opinion is most likely, Yes. Sexting would constitute cheating.