I think CP's offer a good alternative to marriage and it's about time that adults were able to have more control to determine the type of partnership they want to have without the government (law) dictating that only people in a marriage are eligible for certain benefits. It's not a zero sum game, extending CP to all people won't diminish a single group's rights.
Whether you care that a heterosexual couple can enter into a civil partnership or not, the principle of equality means that such unions should be available for all people or no one and I prefer that they become available for all. I personally think they are a good idea for all adults for many reasons. After all, isn't it strange that a bisexual person may enter into a civil partnership with their same sex partner but not if their partner is opposite sex? What if a transgender person transitions and is no longer considered to be legally in a civil partnership that they entered into before transitioning? Limiting civil partnerships just seems bizarre to me and I think they should have been available for all from the very start, of course I know the original purpose was to show minimal acceptance of gay unions.
There are many reasons why a man and a woman may choose a civil partnership over matrimony. Some feminists believe that marriage historically is an institution that has oppressed women. They prefer bonding in a non-marital family unit such as domestic partnership. Then there are women who have divorced due to domestic violence or abuse in a marriage. When they find another mate, they may be reluctant to marry, at least for a few years. They may see domestic partnership as a way of establishing a family unit without surrendering themselves to a spouse through marriage. Given that the UK still does not have "no-fault" divorce, escaping an abusive marriage is harder than it should be.
Many seniors who are widowed and many people of all ages with serious disabilities do not marry because, if they do, they might lose pension survivor benefits or government subsidies. Other seniors are fearful that a new marriage might upset estate plans for their adult children.
There are also religious reasons. Some people whose spouses have died or who have been divorced against their will have vowed never to marry again. To them, marriage is a religious experience they will only have once in life. Being a truly secular institution that does not require a sexual relationship as marriage presumably does, a civil partnership is another matter entirely. Widows & widowers or divorcees may want a civil partnership for purposes of companionship and security without any sexual overtones, and that should be their prerogative.
It's not hard to imagine why the government and religious institutions don't want to encourage civil partnerships for all, they would help adults have more flexibility in how they create and dissolve family units, free from the influence and control of religion and presumably upsetting the traditional family unit model which many in our Tory government no doubt view as sacrosanct...