Yes, it's true. What's more, because it will be part of 'relationship' education rather than sex education you will not have the right to withdraw your child.
There's a FAQ here:
www.gov.uk/government/news/relationships-education-relationships-and-sex-education-rse-and-health-education-faqs
Q: Will my child be taught about LGBT relationships?
A: Pupils should be taught about the society in which they are growing up. These subjects are designed to foster respect for others and for difference, and educate pupils about healthy relationships.
Pupils should receive teaching on LGBT content during their school years. Teaching children about the society that we live in and the different types of loving, healthy relationships that exist can be done in a way that respects everyone. Primary schools are strongly encouraged and enabled to cover LGBT content when teaching about different types of families.
Secondary schools should cover LGBT content in their RSE teaching. RSE should meet the needs of all pupils, whatever their developing sexuality or identity – this should include age-appropriate teaching about different types of relationships in the context of the law.
Statutory guidance:
www.gov.uk/government/publications/relationships-education-relationships-and-sex-education-rse-and-health-education
These are the relevant bits:
Lesbian, Gay, Bisexual and Transgender (LGBT)
36. In teaching Relationships Education and RSE, schools should ensure that the needs of all pupils are appropriately met, and that all pupils understand the importance of equality and respect. Schools must ensure that they comply with the relevant provisions of the Equality Act 2010, (please see The Equality Act 2010 and schools: Departmental advice), under which sexual orientation and gender reassignment are amongst the protected characteristics.
37. Schools should ensure that all of their teaching is sensitive and age appropriate in approach and content. At the point at which schools consider it appropriate to teach their pupils about LGBT, they should ensure that this content is fully integrated into their programmes of study for this area of the curriculum rather than delivered as a stand-alone unit or lesson. Schools are free to determine how they do this, and we expect all pupils to have been taught LGBT content at a timely point as part of this area of the curriculum.
and in the section for secondary schools:
75. Pupils should be taught the facts and the law about sex, sexuality, sexual health and gender identity in an age-appropriate and inclusive way. All pupils should feel that the content is relevant to them and their developing sexuality. Sexual orientation and gender identity should be explored at a timely point and in a clear, sensitive and respectful manner. When teaching about these topics, it must be recognised that young people may be discovering or understanding their sexual orientation or gender identity. There should be an equal opportunity to explore the features of stable and healthy same-sex relationships. This should be integrated appropriately into the RSE programme, rather than addressed separately or in only one lesson.
Schools have a lot of freedom in how they go about meeting these requirements and they are required to consult with parents when designing their curriculum.
It's important to keep tabs on what resources they are using and what external organisations they're bringing in. Some of them are absolutely shocking - really basic things like not including sex in the list of protected characteristics.