Is there a way to transfer long texts to email?

Tap and hold on the text reply you want. On the contextual menu that appears, press Copy. Open a new email and paste it into the body.

If you mean the entire sequence, once again press and hold on any reply. When the contextual menu appears, tap More. Now tap on every empty circle to the left so each contains a check box. When they're all checked, tap the send arrow at the bottom right.

Unfortunately, the latter method only sends a new message with everything you checked. I don't see a way to drop the entire sequence of replies into an email.

Edit: If you have a Mac, this can be done. Open Messages and the sequence you want to send. Click and hold the mouse button down at the top, and then drag across all of the text as you would in any word processor.

Open a new email in Mail and paste the text. If you paste that text into an email other than Apple's Mail app, you may get an unsorted mess, as I did in eM Client. To avoid that, first paste the text in to TextEdit (in Rich Text mode). Then copy/paste it from there into your third party email client.

Edit 2: If there are any photos in the replies, the first photo you drag across will be all you get on a paste. You'll have to copy all of the text down to a photo (without including the photo in the grab) and paste that. Grab the photo and paste that separately. Then continue on underneath the photo with the next batch of text.