A single form view does not require you do this and you can attach a PDF of a layout instead of inserting text into the email body. Those are options you may also want to consider if you haven't already done so.
There are two ways to do what you requested:
Define a calculation field, cTextLine in the portal's table that combines all the data you want into a single line:
Field1 & " " & Field2 & " " & Field3
or you can use:
Field1 & Chr(9) & Field2 & Chr(9) & Field3
To put tab characters between them instead of spaces.
Then You can use list ( PortalTable::cTextLine ) in a calculation field defined in the parent table to pull the text from all the related records into a single list.
Design a layout based on the portal records. Put only the fields you want in the block of text on this layout. Perform a Find or use Go To Related Records to pull up just the records you want in your block of text. Copy All Records can now copy all the data into your clipboard and you can then paste this text into a text field for inclusion in your email. The field data will be separated by tabs and the records will each be separated by returns.