The rambam's laws of isurei bia chapter twelve (https://www.chabad.org/library/article_cdo/aid/960660/jewish/Issurei-Biah-Chapter-Twelve.htm) talk about this issue:
When a Jew engages in relations with a woman from other nations, [taking her] as his wife or a Jewess engages in relations with a non-Jew as his wife, they are punished by lashes, ...
Yes, it is a sin, because you are causing someone else to sin. And that is a sin.
Teshuvah in general is described as stopping to do the sin and to confess before God. So in this case it would mean leaving the Jewish wife.
Highly instructive are the comments of R. Nissim Gaonin his introduction to the ...
If they do not repent, will they not be redeemed at all? Rather, the Holy One, Blessed be He, will establish a king for them whose decrees are as harsh as those issued by Haman, and the Jewish people will have no choice but to repent