It should be obvious from the context that seed here refers to God's seed, not man's seed. That's what being born of God means; that we are of God's seed. It means that God's seed remains in us after we are born of God and bears fruit in virtue (i.e. of not sinning).
If it refers to remaining continent, the whole born of God element that clearly brackets it makes no sense.
There are plenty of other verses in Scripture that refute Gnosticism, e.g.,
2 Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God: 3 And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world."
(1 John 4:2–3 KJV)
1 Timothy 4:1–3:
4 1 Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils; 2 Speaking lies in hypocrisy; having their conscience seared with a hot iron; 3 Forbidding to marry, and commanding to abstain from meats, which God hath created to be received with thanksgiving of them which believe and know the truth.