Ukip stands for the UK Independence Party, an anti-European Union party whose biggest successes have come in EU elections. While I'm sure the party members themselves would argue that they are not xenophobic, they clearly benefit from anti-immigrant sentiment, which in the UK also includes a certain level of anti-looks-like-a-Muslim totally-not-racism-honest.
The party leadership must have been worried, however, that this approach can only go so far. So we have this move, which plans to exploit a whole new, untapped area of bigotry! Clearly this is a smart political strategy.
The issue as a whole is David Cameron's plans to introduce full gay marriage in the UK. The Labour party introduced civil unions in 2004, which offer all rights accrued under marriage to gay couples. Calling the arrangement a civil partnership, rather than marriage, was one of the compromises needed to get the legislation to pass. It's easy to say that they should have gone further, but to illustrate the rapid progress in this area, Section 28, a nasty piece of anti-LGBT legislation from Thatcher's government, had only been repealed the previous year; while the fact that members of the Labour cabinet were gay had been something of a scandal as recently as 1997.
Against this background, Cameron's plans to pass (almost) full marriage equality is commendable. The main gains to be offered relate to titles and, hopefully, recognition with other countries; gay and lesbian couples would still be legally barred from getting married in the Church of England. It's a sign of the efforts Cameron has made to broaden the appeal of the Conservatives; consider the large number of non-white and women candidates he sponsored at the last election. Unfortunately, while Cameron himself is offering a more humane social policy, not everyone in his party is willing to agree. With Ukip naturally appealing to a similar sector of the electorate, they've decided to try and exploit this.