12 Cor6:14 is not speaking of marriage.
Let's look at the relevant verses in the Bible and put them in context.
1 Cor 7:14
For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband: else were your children unclean; but now are they holy.
Paul says the unbeliever is 'sanctified' by the believer.
Since the unbeliever is sanctified by the believer and the marriage becomes HOLY, 2 Cor 6 cannot be speaking of marriage as Paul mentions Belial, idolatry and infidel, and that we should distance ourselves from them. If he had been talking about marriage, he would have been contradicting himself with 1 Cor 7 when he says that if the unbeliever is pleased to remain with the believer, the believer must not separate. Indeed, in 2 Cor 6 :14, marriage is not even mentioned, whereas 1 Cor 7 is all about marriage.
Clearly, if the believer involves himself with other religions, then he/she is no longer a believer. In 2 Cor 6, Paul must be be referring to the behaviour of some at the Corinthian church of associating with unbelievers and accepting and participating in their practices and sacrifices, similar to the behaviour of the Israelites in the past of accepting sacrificial offerings to Baal Peor (Numbers 25 and Psalm 106:28).
Therefore, one is 'equally yoked' if one remains in the Lord and does not become involved with the religious practices of the unbeliever.
At best, one may say that one should exercise caution and be prepared that there may be difficulties in marrying an unbeliever, although Paul also applies this to marriage in general, preferring others to be the same as he is if possible.
Most importantly, 1 Cor 7 was written BEFORE 2 Cor 6. To me, therefore, it is obvious that 2 Cor 6 is not meant to be about 1 Cor 7, but about idolatry instead!
Otherwise Paul would have been made clearer to those spoken to in 1st letter, as by the time the 2nd letter was written, there was sufficient time gap for many in the congregation to have got married- to unbelievers! Paul would not give such advice piecemeal, and if, as others claim, he meant marriage in 2 Cor 6, they would certainly justified in telling him off.
What about the advice to widows? Well, it is just that- advice to widows, otherwise he would have said to 'ALL'. Naturally, having been bereaved, a widow would already have suffered once. Paul is merely thinking of the widow in this regard and that it may be more difficult for her to deal with problems later if they occur, hence his statement that 'she would be happier'. At that time, women were mostly subservient to men (whatever the religion) and more than likely she would have been forced to convert or participate in the practices of the unbelieving husband.
In the OT, a split was demanded, but Paul says that if the unbeliever is pleased to stay with the believer, they are to remain together.
One must ask then, why is a marriage between an unbeliever and believer wrong, if the unbeliever is 'pleased' to marry the believer? Paul, after all, says that if the unbeliever is 'pleased' to stay with the believer, they should remain together?
"My yoke is easy and my burden light". The yoke is obviously us being yoked to Christ. There is an 'unequal yoke' with unbelievers if we also involve ourself with/practice their religions, as we cannot pull both ways. In a marriage, if the unbeliever accepts this and does not hinder the believer from following their religion, and wants to stay, then ok, that person is sanctified through the believer. This is not so if we join in their practice.