The Lib Dem and Labour party manifestos released this week include "extreme' proposals on abortion and transgenderism, Christian Concern has warned.
Both parties have promised sweeping changes to the law, including the introduction of gender self-identification, removing the requirement for a medical diagnosis.
The Lib Dems also plan to recognise "non-binary gender identities" and introduce an 'X' gender option on passports.
Christian Concern warned that the proposals threaten women's rights "in every area".
"There is no scientific basis for 'non-binary gender identities' and no way for them to be assessed," it said.
"These proposals strike at the heart of human nature as male and female and will completely undermine women's rights in every area."
Both parties are backing no-fault divorce, although Labour's proposals go further with plans to extend legal rights to cohabiting couples, and to abolish the marriage tax allowance, plans Christian Concern said risk "further undermining the institution of marriage".
Labour has also pledged to make LGBT+ relationships and sex education mandatory in schools. The proposal follows the deselection of former Labour MP Roger Godsiff after he defended parental rights.
"Labour clearly do not care about parental rights and religious concerns from Christians, Muslims, Jews and others about the sexualisation of young children," Christian Concern said.
On abortion, Labour has promised to completely decriminalise abortion, paving the way for terminations for any reason up to birth, while the Lib Dems are proposing to decriminalise abortion up to 24 weeks.
The Lib Dems have also made a commitment to "fund abortion clinics to provide their services free of charge to service users regardless of nationality or residency", while "working to extend reproductive rights" overseas.
"This will mean that babies in the womb can be killed for any reason with no checks and balances," Christian Concern said.
"This would therefore legalise gender-selective abortion which means that Lib Dems presumably approve of this discriminatory practice.
"While not as extreme as Labour's proposal to decriminalise abortion right up to birth, this would still leave England and Wales with on of the most extreme abortion laws in the world."
In addition to plans for the decriminalisation of abortion, the Lib Dems are also proposing to enforce buffer zones around abortion clinics, which will prevent pro-life campaigners from conducting prayer vigils, protests or offering counselling or support to women considering abortions.
The introduction of the proposal to the party manifesto follows the deselection of former MP Rob Flello, a Catholic, over his views on abortion and same-sex marriage.
Mr Flello previously voted against same-sex marriage in Parliament and had voted in favour of giving women considering abortion a legal right to independent counselling. He also supported an amendment to ban sex-selective abortion.
Commenting on his deselection, Christian Concern said: "This made clear that the Lib Dems will not tolerate people who in conscience cannot support the killing of the unborn."
The Right to Life pro-life campaign group has also been critical of the Lib Dem and Labour pledges on abortion.
"The term 'decriminalise' is a misnomer used by abortion campaigners to describe the removal of almost all current safeguards around abortion," it said.
"Even if the Liberal Democrats retained 'the existing 24-week limit' almost all existing legal safeguards around abortion would be removed prior to 24 weeks."
It added that both parties "should be committing to bringing forward sensible new restrictions and increased support for women with unplanned pregnancies".
"This would ensure we were working together as a society to reduce the tragic number of abortions that happen each year."
In the run-up to the election, it is encouraging pro-life individuals to ask their candidates to take the Both Lives Matter pledge to end sex-selective abortions, commit to lowering the abortion time limit to 12 weeks, and work to end pregnancy and maternity discrimination by supporting policies that encourage women back into the workplace after having children.
